2D Canvas doesn't blend video with the destination buffer.

cc/resources/video_resource_updater.cc

Index: cc/resources/video_resource_updater.cc
diff --git a/cc/resources/video_resource_updater.cc b/cc/resources/video_resource_updater.cc
index a1286b917b4f2bde8af4248eb7654b2da202db4d..f6278266c57ff36bf325cddfb747da4f119197ff 100644
--- a/cc/resources/video_resource_updater.cc
+++ b/cc/resources/video_resource_updater.cc
@@ -249,10 +249,12 @@ VideoFrameExternalResources VideoResourceUpdater::CreateForSoftwarePlanes(
     {
       ResourceProvider::ScopedWriteLockSoftware lock(
           resource_provider_, plane_resources[0].resource_id);
+      // Use Src mode to completely overwrite the reusing buffer.

[dshwang, 2014/08/07 12:47:53]

I change this comment from "Use Src mode to avoid clearing the buffer"
Previous comment is technically wrong. SrcOver mode also doesn't clear the
buffer. The benefit of Src mode is that it doesn't blend src with dest buffer.
It just copy&paste src to dest buffer. compositor needs to use Src mode because
of reducing clearing operation for performance.

       video_renderer_->Paint(video_frame.get(),
                              lock.sk_canvas(),
                              video_frame->visible_rect(),
-                             0xff);
+                             0xff,
+                             SkXfermode::kSrc_Mode);
     }
 
     RecycleResourceData recycle_data = {