Implement one-page-at-a-time mousewheel scrolling in the impl thread.

cc/layer_impl.cc

 // Copyright 2011 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/layer_impl.h"
 
 #include "base/debug/trace_event.h"
 #include "base/stringprintf.h"
 #include "base/values.h"
 #include "cc/animation_registrar.h"
@@ skipping 802 matching lines @@
     if (m_maxScrollOffset == maxScrollOffset)
         return;
     m_maxScrollOffset = maxScrollOffset;
 
     layerTreeImpl()->SetNeedsUpdateDrawProperties();
 
     if (m_scrollbarAnimationController)
         m_scrollbarAnimationController->updateScrollOffset(this);
 }
 
+void LayerImpl::setVisibleScrollArea(gfx::Size visibleScrollArea)
+{
+    if (m_visibleScrollArea == visibleScrollArea)
+        return;
+    m_visibleScrollArea = visibleScrollArea;
+}
+
 ScrollbarLayerImpl* LayerImpl::horizontalScrollbarLayer()
 {
     return m_scrollbarAnimationController ? m_scrollbarAnimationController->horizontalScrollbarLayer() : 0;
 }
 
 const ScrollbarLayerImpl* LayerImpl::horizontalScrollbarLayer() const
 {
     return m_scrollbarAnimationController ? m_scrollbarAnimationController->horizontalScrollbarLayer() : 0;
 }
 
@@ skipping 30 matching lines @@
     if (!m_scrollbarAnimationController) {
         if (m_layerTreeImpl->settings().useLinearFadeScrollbarAnimator)
             m_scrollbarAnimationController = createScrollbarAnimationControllerWithFade(this);
         else
             m_scrollbarAnimationController = ScrollbarAnimationController::create(this);
     }
     m_scrollbarAnimationController->setVerticalScrollbarLayer(scrollbarLayer);
 }
 
 }  // namespace cc