cc: Discard SkCanvas (frame-buffer) before rasterization.

cc/resources/picture_pile_impl.h

 // Copyright 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CC_RESOURCES_PICTURE_PILE_IMPL_H_
 #define CC_RESOURCES_PICTURE_PILE_IMPL_H_
 
 #include <list>
 #include <map>
 #include <set>
@@ skipping 28 matching lines @@
   // measured value over all runs.
   void RasterDirect(
       SkCanvas* canvas,
       const gfx::Rect& canvas_rect,
       float contents_scale,
       RenderingStatsInstrumentation* rendering_stats_instrumentation);
 
   // Similar to the above RasterDirect method, but this is a convenience method
   // for when it is known that the raster is going to an intermediate bitmap
   // that itself will then be blended and thus that a canvas clear is required.
+  // Note that this function may write outside the canvas_rect.
   void RasterToBitmap(
       SkCanvas* canvas,
       const gfx::Rect& canvas_rect,
       float contents_scale,
       RenderingStatsInstrumentation* stats_instrumentation);
 
   // Called when analyzing a tile. We can use AnalysisCanvas as
   // SkDrawPictureCallback, which allows us to early out from analysis.
   void RasterForAnalysis(
       skia::AnalysisCanvas* canvas,
@@ skipping 88 matching lines @@
   // instance.  This member variable must be last so that other member
   // variables have already been initialized and can be clonable.
   const ClonesForDrawing clones_for_drawing_;
 
   DISALLOW_COPY_AND_ASSIGN(PicturePileImpl);
 };
 
 }  // namespace cc
 
 #endif  // CC_RESOURCES_PICTURE_PILE_IMPL_H_