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..c718eb9be9e3515aed6fac57856b1df564af63fc 100644
--- a/cc/resources/rasterizer.h
+++ b/cc/resources/rasterizer.h
@@ -11,17 +11,16 @@
 #include "cc/resources/resource_format.h"
 #include "cc/resources/task_graph_runner.h"
 
-class SkCanvas;
-
 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() {}