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..8ab17f24671bba1fba36e03660951c7adf91132e 100644
--- a/content/browser/renderer_host/render_widget_host_view_mac.mm
+++ b/content/browser/renderer_host/render_widget_host_view_mac.mm
@@ -35,6 +35,7 @@
 #include "content/browser/frame_host/frame_tree_node.h"
 #include "content/browser/frame_host/render_frame_host_impl.h"
 #include "content/browser/gpu/compositor_util.h"
+#include "content/browser/renderer_host/input/mouse_wheel_rails_filter_mac.h"
 #include "content/browser/renderer_host/render_view_host_impl.h"
 #include "content/browser/renderer_host/render_widget_helper.h"
 #import "content/browser/renderer_host/render_widget_host_view_mac_dictionary_helper.h"
@@ -1740,6 +1741,7 @@ void RenderWidgetHostViewMac::OnDisplayMetricsChanged(
     canBeKeyView_ = YES;
     opaque_ = YES;
     focusedPluginIdentifier_ = -1;
+    mouseWheelFilter_.reset(new content::MouseWheelRailsFilterMac);
 
     // OpenGL support:
     if ([self respondsToSelector:
@@ -2255,8 +2257,9 @@ 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 +2381,9 @@ 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);
   }
 }