[Aura] Support transparent webkit.

Support turning on webkit transparency by setting a background image.
- Update RenderWidget::PaintRect to handle background transparency;
- Update BackingStoreSkia::PaintToBackingStore to handle webkit transparency;
- Turn on compositing of RenderWidgetHostViewAura's layer when we set a background with transparency;

BUG=98311
TEST=Verify when applist change (chromium:98308) is in.


Review URL: http://codereview.chromium.org/8369006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@106943 0039d316-1c4b-4281-b951-d872f2087c98

1 files changed, 12 insertions, 1 deletions

diff --git a/content/renderer/render_widget.cc b/content/renderer/render_widget.cc
index b7b1605..97eac4c 100644
--- a/content/renderer/render_widget.cc
+++ b/content/renderer/render_widget.cc
@@ -534,8 +534,19 @@ void RenderWidget::PaintRect(const gfx::Rect& rect,
                                                     SkShader::kRepeat_TileMode,
                                                     SkShader::kRepeat_TileMode);
     paint.setShader(shader)->unref();
-    paint.setXfermodeMode(SkXfermode::kSrcOver_Mode);
+
+    // Use kSrc_Mode to handle background_ transparency properly.
+    paint.setXfermodeMode(SkXfermode::kSrc_Mode);
+
+    // Canvas could contain multiple update rects. Clip to given rect so that
+    // we don't accidentally clear other update rects.
+    canvas->save();
+    SkRect clip;
+    clip.set(SkIntToScalar(rect.x()), SkIntToScalar(rect.y()),
+             SkIntToScalar(rect.right()), SkIntToScalar(rect.bottom()));
+    canvas->clipRect(clip);
     canvas->drawPaint(paint);
+    canvas->restore();
   }
 
   // First see if this rect is a plugin that can paint itself faster.