Mac: Add rails for scrolling.

content/browser/renderer_host/render_widget_host_view_mac.mm

Index: content/browser/renderer_host/render_widget_host_view_mac.mm
diff --git a/content/browser/renderer_host/render_widget_host_view_mac.mm b/content/browser/renderer_host/render_widget_host_view_mac.mm
index 86f5a774225371db85fd1cc8dbf5f64a44489c0e..04eacdc2f09224f483b112ca2c78d089f0f94dc8 100644
--- a/content/browser/renderer_host/render_widget_host_view_mac.mm
+++ b/content/browser/renderer_host/render_widget_host_view_mac.mm
@@ -2255,8 +2255,9 @@ void RenderWidgetHostViewMac::OnDisplayMetricsChanged(
     // Allow rubber-banding in both directions.
     bool canRubberbandLeft = true;
     bool canRubberbandRight = true;
-    const WebMouseWheelEvent webEvent = WebInputEventFactory::mouseWheelEvent(
+    WebMouseWheelEvent webEvent = WebInputEventFactory::mouseWheelEvent(
         event, self, canRubberbandLeft, canRubberbandRight);
+    webEvent.railsMode = mouseWheelFilter_.UpdateRailsMode(webEvent);
     renderWidgetHostView_->render_widget_host_->ForwardWheelEvent(webEvent);
   }
 
@@ -2378,8 +2379,9 @@ void RenderWidgetHostViewMac::OnDisplayMetricsChanged(
   if (renderWidgetHostView_->render_widget_host_) {
     BOOL canRubberbandLeft = [responderDelegate_ canRubberbandLeft:self];
     BOOL canRubberbandRight = [responderDelegate_ canRubberbandRight:self];
-    const WebMouseWheelEvent webEvent = WebInputEventFactory::mouseWheelEvent(
+    WebMouseWheelEvent webEvent = WebInputEventFactory::mouseWheelEvent(
         event, self, canRubberbandLeft, canRubberbandRight);
+    webEvent.railsMode = mouseWheelFilter_.UpdateRailsMode(webEvent);
     renderWidgetHostView_->render_widget_host_->ForwardWheelEvent(webEvent);
   }
 }