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 976f4cb9dbe809e34488e53fe140d20b13d8f5d6..df74707afa7ae64dc5a90a6b614ab4fd5cd08f94 100644
--- a/cc/scrollbar_layer_impl.cc
+++ b/cc/scrollbar_layer_impl.cc
@@ -138,6 +138,11 @@ void ScrollbarLayerImpl::pushPropertiesTo(LayerImpl* layer)
 
 void ScrollbarLayerImpl::appendQuads(QuadSink& quadSink, AppendQuadsData& appendQuadsData)
 {
+    // TODO(wjmaclean) Make quads respect layer opacity for fading scrollbars in/out
+    // via opacity animations.
+    if (!opacity())
+        return;
+
     bool premultipledAlpha = true;
     bool flipped = false;
     gfx::PointF uvTopLeft(0.f, 0.f);