Add viewport scrollbar class to support overlay scrollbars for pinch zoom virtual viewport.

cc/trees/layer_tree_impl.cc

Index: cc/trees/layer_tree_impl.cc
diff --git a/cc/trees/layer_tree_impl.cc b/cc/trees/layer_tree_impl.cc
index f0be76201af145cd48af1ffd973d4e553d9ec389..d87d024ac6e08305a6381c7fd8faf9702340e4c4 100644
--- a/cc/trees/layer_tree_impl.cc
+++ b/cc/trees/layer_tree_impl.cc
@@ -280,6 +280,10 @@ void LayerTreeImpl::UpdateDrawProperties() {
                  "LayerTreeImpl::UpdateDrawProperties",
                  "IsActive",
                  IsActiveTree());
+
+    // TODO(wjmaclean): when settings().use_pinch_virtual_viewport is specified,

[enne (OOO), 2013/06/12 19:44:38]

Maybe LayerTreeHost can be told the id of the page scale layer and this code can
be ignorant about settings and which layer that actually is.

[wjmaclean, 2013/06/12 19:54:36]

Yes, that is the ultimate goal. LTH will be told which layer ids to use for
inner/outer/page layers, and set that here. That's planned for the next CL.

+    // pass the page_scale_layer instead of root_scroll_layer_ for the page
+    // scale application layer in the following call.
     LayerTreeHostCommon::CalculateDrawProperties(
         root_layer(),
         layer_tree_host_impl_->DeviceViewport().size(),