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

cc/layers/layer_impl.cc

Index: cc/layers/layer_impl.cc
diff --git a/cc/layers/layer_impl.cc b/cc/layers/layer_impl.cc
index 7abc6c3787fece894077f4d19f6ebcca5a5b634c..f89786a47f72bb0adfa5385f210918008cee1891 100644
--- a/cc/layers/layer_impl.cc
+++ b/cc/layers/layer_impl.cc
@@ -384,9 +384,12 @@ void LayerImpl::ApplySentScrollDeltas() {
 
 InputHandler::ScrollStatus LayerImpl::TryScroll(
     gfx::PointF screen_space_point,
-    InputHandler::ScrollInputType type) const {
-
-  if (should_scroll_on_main_thread()) {
+    InputHandler::ScrollInputType type,
+    bool main_frame_is_scrollable) const {
+  // The reasons forcing use to scroll on the main thread here are applicable
+  // only for main frame scrolling. If we cannot scroll the main frame, then
+  // we can disregard these reasons.
+  if (should_scroll_on_main_thread() && main_frame_is_scrollable) {
     TRACE_EVENT_INSTANT0("cc,impl-scroll",
         "LayerImpl::TryScroll: Failed ShouldScrollOnMainThread",
         TRACE_EVENT_SCOPE_THREAD);