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

BUG=361266

Review URL: https://codereview.chromium.org/232843006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@269958 0039d316-1c4b-4281-b951-d872f2087c98

cc/resources/picture_pile_impl.cc

@@ -98,6 +98,7 @@ void PicturePileImpl::RasterToBitmap(
     const gfx::Rect& canvas_rect,
     float contents_scale,
     RenderingStatsInstrumentation* rendering_stats_instrumentation) {
+  canvas->discard();
   if (clear_canvas_with_debug_color_) {
     // Any non-painted areas in the content bounds will be left in this color.
     canvas->clear(DebugColors::NonPaintedFillColor());

cc/resources/picture_pile_impl.h

@@ -46,6 +46,7 @@ class CC_EXPORT PicturePileImpl : public PicturePileBase {
   // 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,