cc: Do bitmap conversion for RasterBuffer in the worker thread.

cc/resources/tile_manager.cc

Index: cc/resources/tile_manager.cc
diff --git a/cc/resources/tile_manager.cc b/cc/resources/tile_manager.cc
index f3b15d97eac1f8a536cbd88d37d11a8c050d4802..c645c3b8663b4bbc792bde6ab6bfbd19e5f60699 100644
--- a/cc/resources/tile_manager.cc
+++ b/cc/resources/tile_manager.cc
@@ -130,7 +130,7 @@ class RasterTaskImpl : public RasterTask {
     devtools_instrumentation::ScopedLayerTask layer_task(
         devtools_instrumentation::kRasterTask, layer_id_);
 
-    SkCanvas* canvas = raster_buffer_->AcquireSkCanvas();
+    skia::RefPtr<SkCanvas> canvas = raster_buffer_->AcquireSkCanvas();
     DCHECK(canvas);
 
     skia::RefPtr<SkDrawFilter> draw_filter;
@@ -156,7 +156,8 @@ class RasterTaskImpl : public RasterTask {
     RenderingStatsInstrumentation* stats =
         tile_resolution_ == HIGH_RESOLUTION ? rendering_stats_ : NULL;
     DCHECK(picture_pile);
-    picture_pile->RasterToBitmap(canvas, content_rect_, contents_scale_, stats);
+    picture_pile->RasterToBitmap(
+        canvas.get(), content_rect_, contents_scale_, stats);
 
     if (rendering_stats_->record_rendering_stats()) {
       base::TimeDelta current_rasterize_time =
@@ -169,7 +170,8 @@ class RasterTaskImpl : public RasterTask {
           100);
     }
 
-    raster_buffer_->ReleaseSkCanvas();
+    bool content_has_changed = raster_buffer_->ReleaseSkCanvas(canvas.get());
+    set_content_has_changed(content_has_changed);
   }
 
   PicturePileImpl::Analysis analysis_;