cc: Don't perform image analysis if the DisplayItemList has no images.

cc/paint/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/paint/display_item_list.h"
 
 #include <stddef.h>
 
 #include <string>
 
@@ skipping 508 matching lines @@
     PictureDebugUtil::SerializeAsBase64(picture.get(), &b64_picture);
     state->SetString("skp64", b64_picture);
   }
 
   return state;
 }
 
 void DisplayItemList::GenerateDiscardableImagesMetadata() {
   // This should be only called once.
   DCHECK(image_map_.empty());
+  if (!has_discardable_images_)
+    return;
 
   gfx::Rect bounds = rtree_.GetBounds();
   DiscardableImageMap::ScopedMetadataGenerator generator(
       &image_map_, gfx::Size(bounds.right(), bounds.bottom()));
   auto* canvas = generator.canvas();
   for (const auto& item : items_)
     RasterItem(item, canvas, nullptr);
 }
 
 void DisplayItemList::GetDiscardableImagesInRect(
     const gfx::Rect& rect,
     float contents_scale,
     const gfx::ColorSpace& target_color_space,
     std::vector<DrawImage>* images) {
   image_map_.GetDiscardableImagesInRect(rect, contents_scale,
                                         target_color_space, images);
 }
 
 gfx::Rect DisplayItemList::GetRectForImage(ImageId image_id) const {
   return image_map_.GetRectForImage(image_id);
 }
 
 }  // namespace cc