[2/5] Disable transformed rasterization if layer is opaque

cc/layers/picture_layer.cc

Index: cc/layers/picture_layer.cc
diff --git a/cc/layers/picture_layer.cc b/cc/layers/picture_layer.cc
index 40742e50ea8b845178c2eeeb20a37e59219a473a..6cf58ba6bae3dd6a887a390dd6ce34a7f60d69b4 100644
--- a/cc/layers/picture_layer.cc
+++ b/cc/layers/picture_layer.cc
@@ -103,7 +103,13 @@ bool PictureLayer::Update() {
 
   gfx::Size layer_size = paint_properties().bounds;
 
-  recording_source_->SetBackgroundColor(SafeOpaqueBackgroundColor());
+  // When transformed rasterization is used, we try to match the rendering

[enne (OOO), 2017/03/29 13:01:42]

I'm not sure I agree with this.  I think if a layer is marked as opaque then
that should take priority over transformed rasterization.  In other words,
ShouldUseTransformedRasterization should take into account contents_opaque, and
then this change here wouldn't be needed.

[trchen, 2017/03/29 21:50:36]

I thought layer opaqueness is more of a hint than a directive. i.e. The layer
client is guaranteed to return a paint record that is opaque within the layer
bound. Thus cc could potentially apply optimization like dropping the alpha
channel, using kSrc mode, not clearing raster buffer, that kind of stuff.

Actually I feel background overfill does more harm than good with translated
layers. Maybe I don't fully understand the rationale of it. Do you remember why
was it introduced in the first place?

+  // without a compositing layer, thus we don't over draw background to fill
+  // snapped edges.
+  SkColor background_color = ShouldUseTransformedRasterization()
+                                 ? SK_ColorTRANSPARENT
+                                 : SafeOpaqueBackgroundColor();
+  recording_source_->SetBackgroundColor(background_color);
   recording_source_->SetRequiresClear(
       !contents_opaque() &&
       !picture_layer_inputs_.client->FillsBoundsCompletely());