ui: Cache the output of View::OnPaint when the View isn't invalid.

cc/resources/drawing_display_item.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/drawing_display_item.h"
 
 #include <string>
 
 #include "base/strings/stringprintf.h"
 #include "base/trace_event/trace_event_argument.h"
@@ skipping 44 matching lines @@
       "cullRect",
       base::StringPrintf("[%f,%f,%f,%f]", picture_->cullRect().x(),
                          picture_->cullRect().y(), picture_->cullRect().width(),
                          picture_->cullRect().height()));
   std::string b64_picture;
   PictureDebugUtil::SerializeAsBase64(picture_.get(), &b64_picture);
   array->SetString("skp64", b64_picture);
   array->EndDictionary();
 }
 
+scoped_ptr<DrawingDisplayItem> DrawingDisplayItem::Clone() {
+  return Create(picture_);
+}
+
 }  // namespace cc