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

cc/tree_synchronizer.cc

Index: cc/tree_synchronizer.cc
diff --git a/cc/tree_synchronizer.cc b/cc/tree_synchronizer.cc
index 8c3a3700a8cdc31bb350fbfe69d1e1564709c356..45820dbcf69eddb3e4d98e496ec8210651190689 100644
--- a/cc/tree_synchronizer.cc
+++ b/cc/tree_synchronizer.cc
@@ -91,6 +91,8 @@ scoped_ptr<LayerImpl> synchronizeTreesRecursiveInternal(RawPtrLayerImplMap& newL
     // Remove all dangling pointers. The pointers will be setup later in updateScrollbarLayerPointersRecursive phase
     layerImpl->setHorizontalScrollbarLayer(0);
     layerImpl->setVerticalScrollbarLayer(0);
+    layerImpl->setHorizontalPinchZoomScrollbarLayer(0);
+    layerImpl->setVerticalPinchZoomScrollbarLayer(0);
 
     return layerImpl.Pass();
 }
@@ -126,10 +128,17 @@ void updateScrollbarLayerPointersRecursiveInternal(const RawPtrLayerImplMap& new
     DCHECK(scrollbarLayerImpl);
     DCHECK(scrollLayerImpl);
 
-    if (scrollbarLayer->orientation() == WebKit::WebScrollbar::Horizontal)
-        scrollLayerImpl->setHorizontalScrollbarLayer(scrollbarLayerImpl);
-    else
-        scrollLayerImpl->setVerticalScrollbarLayer(scrollbarLayerImpl);
+    if (scrollbarLayer->isPinchZoomScrollbar()) {
+        if (scrollbarLayer->orientation() == WebKit::WebScrollbar::Horizontal)
+            scrollLayerImpl->setHorizontalPinchZoomScrollbarLayer(scrollbarLayerImpl);
+        else
+            scrollLayerImpl->setVerticalPinchZoomScrollbarLayer(scrollbarLayerImpl);
+    } else {
+        if (scrollbarLayer->orientation() == WebKit::WebScrollbar::Horizontal)
+            scrollLayerImpl->setHorizontalScrollbarLayer(scrollbarLayerImpl);
+        else
+            scrollLayerImpl->setVerticalScrollbarLayer(scrollbarLayerImpl);
+    }
 }
 
 void updateScrollbarLayerPointersRecursive(const RawPtrLayerImplMap& newLayers, Layer* layer)