cc: Do bitmap conversion for RasterBuffer in the worker thread.

cc/resources/rasterizer.h

Index: cc/resources/rasterizer.h
diff --git a/cc/resources/rasterizer.h b/cc/resources/rasterizer.h
index 17cc94ed6fa8b82f1a17e565f59e0847b2743657..003e5e9fbd5ff83ab7b1c52bb274c61a7049f13b 100644
--- a/cc/resources/rasterizer.h
+++ b/cc/resources/rasterizer.h
@@ -17,11 +17,12 @@ namespace cc {
 class ImageDecodeTask;
 class RasterTask;
 class Resource;
+class RasterBuffer;
 
 class CC_EXPORT RasterizerTaskClient {
  public:
-  virtual SkCanvas* AcquireCanvasForRaster(RasterTask* task) = 0;
-  virtual void ReleaseCanvasForRaster(RasterTask* task) = 0;
+  virtual RasterBuffer* AcquireBufferForRaster(RasterTask* task) = 0;
+  virtual void ReleaseBufferForRaster(RasterTask* task) = 0;
 
  protected:
   virtual ~RasterizerTaskClient() {}