Revert of Revert of Implement DisplayList GatherPixelRefs

cc/resources/display_item_list.cc

 // Copyright 2014 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 "cc/resources/display_item_list.h"
 
 #include <string>
 
 #include "base/trace_event/trace_event.h"
 #include "base/trace_event/trace_event_argument.h"
@@ skipping 125 matching lines @@
   return state;
 }
 
 void DisplayItemList::EmitTraceSnapshot() const {
   TRACE_EVENT_OBJECT_SNAPSHOT_WITH_ID(
       TRACE_DISABLED_BY_DEFAULT("cc.debug.picture") ","
       TRACE_DISABLED_BY_DEFAULT("devtools.timeline.picture"),
       "cc::DisplayItemList", this, AsValue());
 }
 
+void DisplayItemList::GatherPixelRefs(const gfx::Size& grid_cell_size) {
+  // This should be only called once, and only after CreateAndCacheSkPicture.
+  DCHECK(picture_);
+  DCHECK(!pixel_refs_);
+  pixel_refs_ = make_scoped_ptr(new PixelRefMap(grid_cell_size));
+  if (!picture_->willPlayBackBitmaps())
+    return;
+
+  pixel_refs_->GatherPixelRefsFromPicture(picture_.get());
+}
 }  // namespace cc