cc: Fix nullptr dereference when capturing skps.

cc/playback/display_list_raster_source.h

 // Copyright 2014 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_PLAYBACK_DISPLAY_LIST_RASTER_SOURCE_H_
 #define CC_PLAYBACK_DISPLAY_LIST_RASTER_SOURCE_H_
 
 #include <stddef.h>
 
 #include <vector>
@@ skipping 18 matching lines @@
   static scoped_refptr<DisplayListRasterSource>
   CreateFromDisplayListRecordingSource(const DisplayListRecordingSource* other,
                                        bool can_use_lcd_text);
 
   // Raster a subrect of this RasterSource into the given canvas. It is
   // assumed that contents_scale has already been applied to this canvas.
   // Writes the total number of pixels rasterized and the time spent
   // rasterizing to the stats if the respective pointer is not nullptr.
   // It is assumed that the canvas passed here will only be rasterized by
   // this raster source via this call.
-  // virtual for testing.
+  //
+  // Virtual for testing.
+  //
+  // Note that this should only be called after the image decode controller has
+  // been set, which happens during commit.
   virtual void PlaybackToCanvas(SkCanvas* canvas,
                                 const gfx::Rect& canvas_bitmap_rect,
                                 const gfx::Rect& canvas_playback_rect,
                                 float contents_scale) const;
 
   // Similar to above, except that the canvas passed here can (or was already)
   // rasterized into by another raster source. That is, it is not safe to clear
   // the canvas or discard its underlying memory.
   void PlaybackToSharedCanvas(SkCanvas* canvas,
                               const gfx::Rect& canvas_rect,
@@ skipping 104 matching lines @@
                                   const gfx::Rect& canvas_bitmap_rect,
                                   const gfx::Rect& canvas_playback_rect,
                                   float contents_scale) const;
 
   DISALLOW_COPY_AND_ASSIGN(DisplayListRasterSource);
 };
 
 }  // namespace cc
 
 #endif  // CC_PLAYBACK_DISPLAY_LIST_RASTER_SOURCE_H_