Revert of Preserve viewport scrolling layer when a fling starts (patchset #2 id:20001 of https://codereview.chromium.org/1225693008/ )

Reason for revert:
Causes issues with WebView when the embedder overrides computeScroll.

Original issue's description:
> Preserve viewport scrolling layer when a fling starts
>
> A touch fling will lock the scrolling layer to the first layer that
> scrolls after the animation begins. This can disrupt viewport scrolling
> if the outer viewport was the originally targetted scroll layer, e.g.,
> by resetting bookkeeping for scrollbar fade animations. Prevent this
> by allowing scroll bubbling if the viewport is the locked layer when
> the fling starts.
>
> BUG=507786
> CQ_INCLUDE_TRYBOTS=tryserver.blink:linux_blink_rel
>
> Committed: https://crrev.com/89192797b50f7e33967445095cd7fa16b78f68e1
> Cr-Commit-Position: refs/heads/master@{#337747}

TBR=aelias@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=507786,516362

Review URL: https://codereview.chromium.org/1280113003

Cr-Commit-Position: refs/heads/master@{#342514}

2 files changed, 3 insertions, 15 deletions

diff --git a/cc/trees/layer_tree_host_impl.cc b/cc/trees/layer_tree_host_impl.cc
index 628fc59..38d903e 100644
--- a/cc/trees/layer_tree_host_impl.cc
+++ b/cc/trees/layer_tree_host_impl.cc
@@ -2829,15 +2829,12 @@ void LayerTreeHostImpl::ScrollEnd() {
 }
 
 InputHandler::ScrollStatus LayerTreeHostImpl::FlingScrollBegin() {
-  if (!CurrentlyScrollingLayer())
+  if (!active_tree_->CurrentlyScrollingLayer())
     return SCROLL_IGNORED;
 
-  bool currently_scrolling_viewport =
-      CurrentlyScrollingLayer() == OuterViewportScrollLayer() ||
-      CurrentlyScrollingLayer() == InnerViewportScrollLayer();
-  if (!wheel_scrolling_ && !currently_scrolling_viewport) {
+  if (!wheel_scrolling_) {
     // Allow the fling to lock to the first layer that moves after the initial
-    // fling |ScrollBy()| event, unless we're already scrolling the viewport.
+    // fling |ScrollBy()| event.
     did_lock_scrolling_layer_ = false;
     should_bubble_scrolls_ = false;
   }
diff --git a/cc/trees/layer_tree_host_impl_unittest.cc b/cc/trees/layer_tree_host_impl_unittest.cc
index b8dbc1e..a8f0324 100644
--- a/cc/trees/layer_tree_host_impl_unittest.cc
+++ b/cc/trees/layer_tree_host_impl_unittest.cc
@@ -7393,17 +7393,13 @@ TEST_F(LayerTreeHostImplVirtualViewportTest, FlingScrollBubblesToInner) {
     // Make sure the fling goes to the outer viewport first
     EXPECT_EQ(InputHandler::SCROLL_STARTED,
               host_impl_->ScrollBegin(gfx::Point(), InputHandler::GESTURE));
-    EXPECT_EQ(outer_scroll, host_impl_->CurrentlyScrollingLayer());
     EXPECT_EQ(InputHandler::SCROLL_STARTED, host_impl_->FlingScrollBegin());
-    EXPECT_EQ(outer_scroll, host_impl_->CurrentlyScrollingLayer());
 
     gfx::Vector2d scroll_delta(inner_viewport.width(), inner_viewport.height());
     host_impl_->ScrollBy(gfx::Point(), scroll_delta);
     outer_expected += gfx::Vector2dF(scroll_delta.x(), scroll_delta.y());
-    EXPECT_EQ(host_impl_->CurrentlyScrollingLayer(), outer_scroll);
 
     host_impl_->ScrollEnd();
-    EXPECT_EQ(nullptr, host_impl_->CurrentlyScrollingLayer());
 
     EXPECT_VECTOR_EQ(inner_expected, inner_scroll->CurrentScrollOffset());
     EXPECT_VECTOR_EQ(outer_expected, outer_scroll->CurrentScrollOffset());
@@ -7411,20 +7407,15 @@ TEST_F(LayerTreeHostImplVirtualViewportTest, FlingScrollBubblesToInner) {
     // Fling past the outer viewport boundry, make sure inner viewport scrolls.
     EXPECT_EQ(InputHandler::SCROLL_STARTED,
               host_impl_->ScrollBegin(gfx::Point(), InputHandler::GESTURE));
-    EXPECT_EQ(outer_scroll, host_impl_->CurrentlyScrollingLayer());
     EXPECT_EQ(InputHandler::SCROLL_STARTED, host_impl_->FlingScrollBegin());
-    EXPECT_EQ(outer_scroll, host_impl_->CurrentlyScrollingLayer());
 
     host_impl_->ScrollBy(gfx::Point(), scroll_delta);
     outer_expected += gfx::Vector2dF(scroll_delta.x(), scroll_delta.y());
-    EXPECT_EQ(outer_scroll, host_impl_->CurrentlyScrollingLayer());
 
     host_impl_->ScrollBy(gfx::Point(), scroll_delta);
     inner_expected += gfx::Vector2dF(scroll_delta.x(), scroll_delta.y());
-    EXPECT_EQ(outer_scroll, host_impl_->CurrentlyScrollingLayer());
 
     host_impl_->ScrollEnd();
-    EXPECT_EQ(nullptr, host_impl_->CurrentlyScrollingLayer());
 
     EXPECT_VECTOR_EQ(inner_expected, inner_scroll->CurrentScrollOffset());
     EXPECT_VECTOR_EQ(outer_expected, outer_scroll->CurrentScrollOffset());