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

cc/layer_tree_impl.cc

Index: cc/layer_tree_impl.cc
diff --git a/cc/layer_tree_impl.cc b/cc/layer_tree_impl.cc
index 38fb937ccc189ff9802cd1e27d62ccdd85392826..1e2c591a20f09e8a699a9e90ecfa750902ed6d53 100644
--- a/cc/layer_tree_impl.cc
+++ b/cc/layer_tree_impl.cc
@@ -116,9 +116,9 @@ void LayerTreeImpl::UpdateDrawProperties() {
   if (!RootLayer())
     return;
 
-  if (root_scroll_layer()) {
-    root_scroll_layer()->setImplTransform(
-        layer_tree_host_impl_->implTransform());
+  for (size_t i = 0; i < RootLayer()->children().size(); ++i) {
+    LayerImpl* layerImpl = RootLayer()->children()[i];
+    layerImpl->setImplTransform(layer_tree_host_impl_->implTransform());

[Ian Vollick, 2013/01/15 20:33:48]

Please add a comment here that the implTransform is a generalization of page
scale for pinch zoom that includes translation, and that pinch zoom is, by
design, supposed to affect the scale of the main frame's scrollbars so we need
to apply it to all descendants. It's also worth noting here that you can't just
apply it to the root since the root makes assumptions about never being
translated.

   }
 
   {