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

cc/trees/layer_tree_host_common.h

Index: cc/trees/layer_tree_host_common.h
diff --git a/cc/trees/layer_tree_host_common.h b/cc/trees/layer_tree_host_common.h
index 05affee8f31d58da72b20647ce06fc1736726711..19ca98841eddc8d774cd9c5bfe147e6064f2e445 100644
--- a/cc/trees/layer_tree_host_common.h
+++ b/cc/trees/layer_tree_host_common.h
@@ -118,6 +118,9 @@ class CC_EXPORT LayerTreeHostCommon {
   template <typename LayerType>
   static LayerType* FindLayerInSubtree(LayerType* root_layer, int layer_id);
 
+  template <typename LayerType>
+  static LayerType* FindFirstScrollableLayer(LayerType* root_layer);
+
   static Layer* get_child_as_raw_ptr(
       const LayerList& children,
       size_t index) {
@@ -184,6 +187,24 @@ LayerType* LayerTreeHostCommon::FindLayerInSubtree(LayerType* root_layer,
 }
 
 template <typename LayerType>
+LayerType* LayerTreeHostCommon::FindFirstScrollableLayer(LayerType* layer) {
+  if (!layer)
+    return NULL;
+
+  if (layer->scrollable())
+    return layer;
+
+  for (size_t i = 0; i < layer->children().size(); ++i) {
+    LayerType* current = get_child_as_raw_ptr(layer->children(), i);
+    LayerType* found = FindFirstScrollableLayer(current);
+    if (found)
+      return found;
+  }
+
+  return NULL;
+}
+
+template <typename LayerType>
 void LayerTreeHostCommon::CallFunctionForSubtree(
     LayerType* root_layer,
     const base::Callback<void(LayerType* layer)>& function) {