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

cc/trees/layer_tree_host_impl.cc

Index: cc/trees/layer_tree_host_impl.cc
diff --git a/cc/trees/layer_tree_host_impl.cc b/cc/trees/layer_tree_host_impl.cc
index 7d02db56c019a1893502f3e71f3dbcbb98e0b9ba..1f19e9c496e16747743ac21534e080b3a61f046d 100644
--- a/cc/trees/layer_tree_host_impl.cc
+++ b/cc/trees/layer_tree_host_impl.cc
@@ -1807,6 +1807,16 @@ InputHandler::ScrollStatus LayerTreeHostImpl::ScrollBegin(
   LayerImpl* layer_impl = LayerTreeHostCommon::FindLayerThatIsHitByPoint(
       device_viewport_point, active_tree_->RenderSurfaceLayerList());
 
+  LayerImpl* root_scroll_layer =
+      LayerTreeHostCommon::FindFirstScrollableLayer(
+          active_tree_->root_layer());
+
+  bool main_frame_is_scrollable =
+      root_scroll_layer &&
+      root_scroll_layer->scrollable() &&
+      (root_scroll_layer->max_scroll_offset().x() != 0 ||
+       root_scroll_layer->max_scroll_offset().y() != 0);
+
   // Walk up the hierarchy and look for a scrollable layer.
   LayerImpl* potentially_scrolling_layer_impl = 0;
 
@@ -1815,10 +1825,11 @@ InputHandler::ScrollStatus LayerTreeHostImpl::ScrollBegin(
         "impl-scroll", "LayerTreeHostImpl::ScrollBegin considering layer",
         TRACE_EVENT_SCOPE_THREAD,
         "layer_id", layer_impl->id());
-
     // The content layer can also block attempts to scroll outside the main
     // thread.
-    ScrollStatus status = layer_impl->TryScroll(device_viewport_point, type);
+    ScrollStatus status = layer_impl->TryScroll(device_viewport_point,
+                                                type,
+                                                main_frame_is_scrollable);
     if (status == ScrollOnMainThread) {
       rendering_stats_instrumentation_->IncrementMainThreadScrolls();
       TRACE_EVENT_INSTANT0(
@@ -1840,7 +1851,9 @@ InputHandler::ScrollStatus LayerTreeHostImpl::ScrollBegin(
         TRACE_EVENT_SCOPE_THREAD,
         "layer_id", scroll_layer_impl->id());
 
-    status = scroll_layer_impl->TryScroll(device_viewport_point, type);
+    status = scroll_layer_impl->TryScroll(device_viewport_point,
+                                          type,
+                                          main_frame_is_scrollable);
 
     // If any layer wants to divert the scroll event to the main thread, abort.
     if (status == ScrollOnMainThread) {