cc: Use occlusion for deciding raster/eviction priorities in browser

The browser compositor should use occlusion to allow lots of
overlapping windows to take up less memory. This reduces our memory
consumption in ash a bit.

Though, the window UI is not considered opaque currently, so more
memory is still used than could be otherwise.

R=enne, vmpstr
CQ_INCLUDE_TRYBOTS=tryserver.blink:linux_blink_rel

Review URL: https://codereview.chromium.org/1277053002

Cr-Commit-Position: refs/heads/master@{#342493}

2 files changed, 6 insertions, 3 deletions

diff --git a/cc/tiles/prioritized_tile.cc b/cc/tiles/prioritized_tile.cc
index 67f33c9..2e0246c 100644
--- a/cc/tiles/prioritized_tile.cc
+++ b/cc/tiles/prioritized_tile.cc
@@ -33,6 +33,7 @@ void PrioritizedTile::AsValueInto(base::trace_event::TracedValue* value) const {
 
   value->BeginDictionary("combined_priority");
   priority().AsValueInto(value);
+  value->SetBoolean("is_occluded", is_occluded_);
   value->EndDictionary();
 
   value->SetString("resolution", TileResolutionToString(priority().resolution));
diff --git a/ui/compositor/compositor.cc b/ui/compositor/compositor.cc
index b5474a7..42e51cb 100644
--- a/ui/compositor/compositor.cc
+++ b/ui/compositor/compositor.cc
@@ -85,10 +85,12 @@ Compositor::Compositor(gfx::AcceleratedWidget widget,
 
   cc::LayerTreeSettings settings;
 
-  // When impl-side painting is enabled, this will ensure PictureLayers always
-  // can have LCD text, to match the previous behaviour with ContentLayers,
-  // where LCD-not-allowed notifications were ignored.
+  // This will ensure PictureLayers always can have LCD text, to match the
+  // previous behaviour with ContentLayers, where LCD-not-allowed notifications
+  // were ignored.
   settings.layers_always_allowed_lcd_text = true;
+  // Use occlusion to allow more overlapping windows to take less memory.
+  settings.use_occlusion_for_tile_prioritization = true;
   settings.renderer_settings.refresh_rate =
       context_factory_->DoesCreateTestContexts() ? kTestRefreshRate
                                                  : kDefaultRefreshRate;