cc: Optimize GenerateDiscardableImagesMetadata for display items.

cc/playback/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/playback/display_item_list.h"
 
 #include <stddef.h>
 
 #include <string>
 
@@ skipping 229 matching lines @@
           DisplayItemsTracingEnabled()));
 }
 
 void DisplayItemList::GenerateDiscardableImagesMetadata() {
   // This should be only called once.
   DCHECK(image_map_.empty());
 
   gfx::Rect bounds = rtree_.GetBounds();
   DiscardableImageMap::ScopedMetadataGenerator generator(
       &image_map_, gfx::Size(bounds.right(), bounds.bottom()));
-  Raster(generator.canvas(), nullptr, gfx::Rect(), 1.f);
+  auto* canvas = generator.canvas();
+  for (const auto& item : inputs_.items)
+    item.Raster(canvas, nullptr);
 }
 
 void DisplayItemList::GetDiscardableImagesInRect(
     const gfx::Rect& rect,
     float contents_scale,
     std::vector<DrawImage>* images) {
   image_map_.GetDiscardableImagesInRect(rect, contents_scale, images);
 }
 
 gfx::Rect DisplayItemList::GetRectForImage(ImageId image_id) const {
   return image_map_.GetRectForImage(image_id);
 }
 
 }  // namespace cc