cc: Add a rails argument to InputHandler::ScrollBy

cc/input/input_handler.h

Index: cc/input/input_handler.h
diff --git a/cc/input/input_handler.h b/cc/input/input_handler.h
index 81d91b871432f8500a6b9b46834e72c8a11c4d7d..f5b293237959006fe7e627edd59ab42918903e80 100644
--- a/cc/input/input_handler.h
+++ b/cc/input/input_handler.h
@@ -74,6 +74,11 @@ class CC_EXPORT InputHandler {
     ScrollStatusCount
   };
   enum ScrollInputType { GESTURE, WHEEL, NON_BUBBLING_GESTURE };
+  enum RailsMode {
+    RAILS_MODE_FREE,
+    RAILS_MODE_HORIZONTAL,
+    RAILS_MODE_VERTICAL
+  };
 
   // Binds a client to this handler to receive notifications. Only one client
   // can be bound to an InputHandler. The client must live at least until the
@@ -103,9 +108,9 @@ class CC_EXPORT InputHandler {
   // the root overscroll accumulated within this ScrollBegin() scope is reported
   // in the return value's |accumulated_overscroll| field.
   // Should only be called if ScrollBegin() returned SCROLL_STARTED.
-  virtual InputHandlerScrollResult ScrollBy(
-      const gfx::Point& viewport_point,
-      const gfx::Vector2dF& scroll_delta) = 0;
+  virtual InputHandlerScrollResult ScrollBy(const gfx::Point& viewport_point,
+                                            const gfx::Vector2dF& scroll_delta,
+                                            RailsMode rails_mode) = 0;
 
   virtual bool ScrollVerticallyByPage(const gfx::Point& viewport_point,
                                       ScrollDirection direction) = 0;