Implement pinch-zoom scaling for main-frame scrollbars and pinch-zoom overlay scrollbars.

cc/layer.cc

Index: cc/layer.cc
diff --git a/cc/layer.cc b/cc/layer.cc
index 07c811b42256e2792d8ecf842316f469fd7e4e37..c5786d268e838a3175451e9fbd82218319ae3f49 100644
--- a/cc/layer.cc
+++ b/cc/layer.cc
@@ -874,6 +874,12 @@ void Layer::notifyAnimationFinished(double wallClockTime)
         m_layerAnimationDelegate->notifyAnimationFinished(wallClockTime);
 }
 
+void Layer::notifyAnimationPropertyUpdate(const AnimationEvent& event)
+{
+    if (event.targetProperty == Animation::Opacity)
+        setOpacity(event.value);
+}
+
 void Layer::addLayerAnimationEventObserver(LayerAnimationEventObserver* animationObserver)
 {
     if (!m_layerAnimationObservers.HasObserver(animationObserver))