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

cc/scrollbar_layer.cc

Index: cc/scrollbar_layer.cc
diff --git a/cc/scrollbar_layer.cc b/cc/scrollbar_layer.cc
index ded7bf12a9a4f3f7c99066ef7cc33565ab0dc89c..d20de9436b64d927908fc806a9072315e04cc234 100644
--- a/cc/scrollbar_layer.cc
+++ b/cc/scrollbar_layer.cc
@@ -64,6 +64,11 @@ WebKit::WebScrollbar::Orientation ScrollbarLayer::orientation() const
     return m_scrollbar->orientation();
 }
 
+bool ScrollbarLayer::isOverlay() const
+{
+    return m_scrollbar->isOverlay();
+}
+
 int ScrollbarLayer::maxTextureSize() {
     DCHECK(layerTreeHost());
     return layerTreeHost()->rendererCapabilities().maxTextureSize;