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

cc/resources/picture_pile_impl.cc

 // 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.
 
 #include <algorithm>
 #include <limits>
 
 #include "base/debug/trace_event.h"
 #include "cc/base/region.h"
 #include "cc/debug/debug_colors.h"
@@ skipping 80 matching lines @@
     RenderingStatsInstrumentation* stats_instrumentation) {
   RasterCommon(
       canvas, canvas, canvas_rect, contents_scale, stats_instrumentation, true);
 }
 
 void PicturePileImpl::RasterToBitmap(
     SkCanvas* canvas,
     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());
   }
 
   // If this picture has opaque contents, it is guaranteeing that it will
   // draw an opaque rect the size of the layer.  If it is not, then we must
   // clear this canvas ourselves.
   if (contents_opaque_ || contents_fill_bounds_completely_) {
     // Even if completely covered, for rasterizations that touch the edge of the
@@ skipping 341 matching lines @@
        ++it) {
     Picture* picture = it->second.GetPicture();
     if (picture && (processed_pictures.count(picture) == 0)) {
       picture->EmitTraceSnapshot();
       processed_pictures.insert(picture);
     }
   }
 }
 
 }  // namespace cc