cc: Partial tile update for one-copy raster.

cc/playback/raster_source.h

Index: cc/playback/raster_source.h
diff --git a/cc/playback/raster_source.h b/cc/playback/raster_source.h
index c0a484aa8d9a28d312bd8353cbeb402aa7c5d169..8d0356fae3807ecf5abb2331cd3a33d1b7a95284 100644
--- a/cc/playback/raster_source.h
+++ b/cc/playback/raster_source.h
@@ -41,7 +41,8 @@ class CC_EXPORT RasterSource : public base::RefCountedThreadSafe<RasterSource> {
   // It is assumed that the canvas passed here will only be rasterized by
   // this raster source via this call.
   virtual void PlaybackToCanvas(SkCanvas* canvas,
-                                const gfx::Rect& canvas_rect,
+                                const gfx::Rect& canvas_bitmap_rect,
+                                const gfx::Rect& canvas_playback_rect,

[reveman, 2015/05/28 05:07:24]

Instead of adding another rect here, can we use SkCanvas::getBaseLayerSize() to
determine if playback_rect represents the whole canvas and optimized clear
functions can be used?

[danakj, 2015/05/28 18:54:46]

I don't like this, it really limits what the caller can do with the SkCanvas. It
could have a larger canvas (a 256x256 canvas?) for a smaller tile (an edge
tile?).

[reveman, 2015/05/29 14:50:26]

I'm sure you're right. It's just hard for me to see why canvas+rect is not
enough to satisfy both these use cases and canvas+rect+subrect is necessary. I
thought this was what PlaybackToSharedCanvas is already doing (playback into a
larger canvas that already contains some content) and I would have expected the
SharedCanvas version to be used for partial updates based on the description of
that function. Is the additional rect just so we can compute "bool
partial_update = canvas_bitmap_rect != canvas_playback_rect;" in
::PrepareForPlaybackToCanvas or also something else that I'm failing to see?

                                 float contents_scale) const = 0;
 
   // Similar to above, except that the canvas passed here can (or was already)