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

cc/scrollbar_layer_impl.cc

Index: cc/scrollbar_layer_impl.cc
diff --git a/cc/scrollbar_layer_impl.cc b/cc/scrollbar_layer_impl.cc
index 25d51cafd5e5142a132b6bb66fb0e1b95314bb83..8c30bfb88dc6e9c34eecd28ea6e8c44d326f2b90 100644
--- a/cc/scrollbar_layer_impl.cc
+++ b/cc/scrollbar_layer_impl.cc
@@ -41,6 +41,7 @@ ScrollbarLayerImpl::ScrollbarLayerImpl(LayerTreeImpl* treeImpl, int id, scoped_p
     , m_enabled(false)
     , m_isCustomScrollbar(false)
     , m_isOverlayScrollbar(false)
+    , m_isPinchZoomScrollbar(false)
 {
 }
 
@@ -136,6 +137,16 @@ void ScrollbarLayerImpl::pushPropertiesTo(LayerImpl* layer)
 
 void ScrollbarLayerImpl::appendQuads(QuadSink& quadSink, AppendQuadsData& appendQuadsData)
 {
+    if (isPinchZoomScrollbar()) {
+        LayerImpl* rootScrollLayer = layerTreeImpl()->RootScrollLayer();
+        LayerImpl* currentlyScrollingLayer = layerTreeImpl()->CurrentlyScrollingLayer();
+        bool pinchZoomScrollbarShouldDraw = rootScrollLayer &&
+                                            rootScrollLayer == currentlyScrollingLayer &&
+                                            layerTreeImpl()->total_page_scale_factor() > 1;
+        if (!pinchZoomScrollbarShouldDraw)
+            return;
+    }
+
     bool premultipledAlpha = false;
     bool flipped = false;
     gfx::PointF uvTopLeft(0.f, 0.f);