cc: Add a rails argument to InputHandler::ScrollBy

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 00ae766155f380cb6f9ec597dd5b8575d7791b36..9e93c71a7aa4efe58b931595e224a33312c54049 100644
--- a/cc/trees/layer_tree_host_impl.cc
+++ b/cc/trees/layer_tree_host_impl.cc
@@ -2593,12 +2593,17 @@ gfx::Vector2dF LayerTreeHostImpl::ScrollLayer(LayerImpl* layer_impl,
 
 InputHandlerScrollResult LayerTreeHostImpl::ScrollBy(
     const gfx::Point& viewport_point,
-    const gfx::Vector2dF& scroll_delta) {
+    const gfx::Vector2dF& unrailed_scroll_delta,
+    InputHandler::RailsMode rails_mode) {
   TRACE_EVENT0("cc", "LayerTreeHostImpl::ScrollBy");
   if (!CurrentlyScrollingLayer())
     return InputHandlerScrollResult();
 
-  gfx::Vector2dF pending_delta = scroll_delta;
+  gfx::Vector2dF pending_delta = unrailed_scroll_delta;
+  if (rails_mode == InputHandler::RAILS_MODE_HORIZONTAL)
+    pending_delta.set_y(0);
+  if (rails_mode == InputHandler::RAILS_MODE_VERTICAL)
+    pending_delta.set_x(0);
   gfx::Vector2dF unused_root_delta;
   bool did_scroll_x = false;
   bool did_scroll_y = false;