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

cc/paint/display_item_list.cc

Index: cc/paint/display_item_list.cc
diff --git a/cc/paint/display_item_list.cc b/cc/paint/display_item_list.cc
index c1b069548409f6a31a1aeea75870222fc5c62645..432b0baf840a8690c097e6eae68f3ecc9aebdaa0 100644
--- a/cc/paint/display_item_list.cc
+++ b/cc/paint/display_item_list.cc
@@ -262,6 +262,8 @@ void DisplayItemList::Finalize() {
     // This clears both the vector and the vector's capacity, since
     // visual_rects won't be used anymore.
     std::vector<gfx::Rect>().swap(visual_rects_);
+
+  GenerateDiscardableImagesMetadata();
 }
 
 bool DisplayItemList::IsSuitableForGpuRasterization() const {
@@ -526,6 +528,8 @@ DisplayItemList::CreateTracedValue(bool include_items) const {
 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(