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 3d83aebf30f541d6036e683f2ef32d9363dd2fa7..e6be5352133cde8875121472224d8e63ac6bf133 100644
--- a/cc/scrollbar_layer.cc
+++ b/cc/scrollbar_layer.cc
@@ -61,6 +61,10 @@ void ScrollbarLayer::SetScrollLayerId(int id) {
   SetNeedsFullTreeSync();
 }
 
+bool ScrollbarLayer::DoNotSkipOnZeroOpacity() const {
+  return scroll_layer_id_ == Layer::PINCH_ZOOM_ROOT_SCROLL_LAYER_ID;
+}
+
 WebKit::WebScrollbar::Orientation ScrollbarLayer::Orientation() const {
   return scrollbar_->orientation();
 }