cc: Fix nullptr dereference when capturing skps.

cc/playback/display_list_raster_source.h

Index: cc/playback/display_list_raster_source.h
diff --git a/cc/playback/display_list_raster_source.h b/cc/playback/display_list_raster_source.h
index 73c7a7f50ac6c37b7bbba5a2ced37c99d53a6e66..9ebfd4fb657d67b27f54b37cde5f26453b16a3fa 100644
--- a/cc/playback/display_list_raster_source.h
+++ b/cc/playback/display_list_raster_source.h
@@ -36,7 +36,11 @@ class CC_EXPORT DisplayListRasterSource
   // 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,