Ignore should-scroll-on-main-thread if main frame is not scrollable

cc/trees/layer_tree_host.cc

Index: cc/trees/layer_tree_host.cc
diff --git a/cc/trees/layer_tree_host.cc b/cc/trees/layer_tree_host.cc
index ea87e3041b5703329e2ac492b8465715bca56014..d362a6dc31bc1ca298d180893026fa73c9c8c1c7 100644
--- a/cc/trees/layer_tree_host.cc
+++ b/cc/trees/layer_tree_host.cc
@@ -690,22 +690,6 @@ bool LayerTreeHost::UpdateLayers(ResourceUpdateQueue* queue,
   return UpdateLayers(root_layer(), queue);
 }
 
-static Layer* FindFirstScrollableLayer(Layer* layer) {
-  if (!layer)
-    return NULL;
-
-  if (layer->scrollable())
-    return layer;
-
-  for (size_t i = 0; i < layer->children().size(); ++i) {
-    Layer* found = FindFirstScrollableLayer(layer->children()[i].get());
-    if (found)
-      return found;
-  }
-
-  return NULL;
-}
-
 void LayerTreeHost::CalculateLCDTextMetricsCallback(Layer* layer) {
   if (!layer->SupportsLCDText())
     return;
@@ -733,7 +717,8 @@ bool LayerTreeHost::UpdateLayers(Layer* root_layer,
   {
     UpdateHudLayer();
 
-    Layer* root_scroll = FindFirstScrollableLayer(root_layer);
+    Layer* root_scroll =
+        LayerTreeHostCommon::FindFirstScrollableLayer(root_layer);
 
     if (hud_layer_) {
       hud_layer_->PrepareForCalculateDrawProperties(
@@ -1008,7 +993,8 @@ void LayerTreeHost::ApplyScrollAndScale(const ScrollAndScaleSet& info) {
   }
 
   gfx::Vector2d root_scroll_delta;
-  Layer* root_scroll_layer = FindFirstScrollableLayer(root_layer_.get());
+  Layer* root_scroll_layer =
+      LayerTreeHostCommon::FindFirstScrollableLayer(root_layer_.get());
 
   for (size_t i = 0; i < info.scrolls.size(); ++i) {
     Layer* layer =
@@ -1027,7 +1013,8 @@ void LayerTreeHost::ApplyScrollAndScale(const ScrollAndScaleSet& info) {
   if (!root_scroll_delta.IsZero() || info.page_scale_delta != 1.f) {
     // SetScrollOffsetFromImplSide above could have destroyed the tree,
     // so re-get this layer before doing anything to it.
-    root_scroll_layer = FindFirstScrollableLayer(root_layer_.get());
+    root_scroll_layer =
+        LayerTreeHostCommon::FindFirstScrollableLayer(root_layer_.get());
 
     // Preemptively apply the scroll offset and scale delta here before sending
     // it to the client.  If the client comes back and sets it to the same