Implement CSSOM smooth scroll for Elements

Source/core/page/PageAnimator.cpp

 // 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 "config.h"
 #include "core/page/PageAnimator.h"
 
 #include "core/animation/DocumentAnimations.h"
 #include "core/frame/FrameView.h"
 #include "core/frame/LocalFrame.h"
@@ skipping 32 matching lines @@
     WillBeHeapVector<RefPtrWillBeMember<Document>> documents;
     for (Frame* frame = m_page->mainFrame(); frame; frame = frame->tree().traverseNext()) {
         if (frame->isLocalFrame())
             documents.append(toLocalFrame(frame)->document());
     }
 
     for (size_t i = 0; i < documents.size(); ++i) {
         if (documents[i]->frame()) {
             documents[i]->view()->serviceScrollAnimations(monotonicAnimationStartTime);
 
-            if (const FrameView::ScrollableAreaSet* scrollableAreas = documents[i]->view()->scrollableAreas()) {
-                for (ScrollableArea* scrollableArea : *scrollableAreas)
+            if (const FrameView::ScrollableAreaSet* animatingScrollableAreas = documents[i]->view()->animatingScrollableAreas()) {
+                // Iterate over a copy, since ScrollableAreas may deregister
+                // themselves during the iteration.
+                Vector<ScrollableArea*> animatingScrollableAreasCopy;
+                copyToVector(*animatingScrollableAreas, animatingScrollableAreasCopy);
+                for (ScrollableArea* scrollableArea : animatingScrollableAreasCopy)
                     scrollableArea->serviceScrollAnimations(monotonicAnimationStartTime);
             }
         }
     }
 
     for (size_t i = 0; i < documents.size(); ++i) {
         DocumentAnimations::updateAnimationTimingForAnimationFrame(*documents[i], monotonicAnimationStartTime);
         SVGDocumentExtensions::serviceOnAnimationFrame(*documents[i], monotonicAnimationStartTime);
     }
 
@@ skipping 30 matching lines @@
     // setFrameRect(). This will be a quick operation for most frames, but the
     // NativeWindowWidgets will update a proper clipping region.
     view->setFrameRect(view->frameRect());
 
     // setFrameRect may have the side-effect of causing existing page layout to
     // be invalidated, so layout needs to be called last.
     view->updateLayoutAndStyleForPainting();
 }
 
 }