Add rails to input wheel events.

public/web/WebInputEvent.h

Index: public/web/WebInputEvent.h
diff --git a/public/web/WebInputEvent.h b/public/web/WebInputEvent.h
index a21e26705b7e3b47fe8f4e50d5a4a3b589a450b8..058300bf7a27bc49809e40095da7ca0b77589c38 100644
--- a/public/web/WebInputEvent.h
+++ b/public/web/WebInputEvent.h
@@ -343,6 +343,15 @@ protected:
 
 class WebMouseWheelEvent : public WebMouseEvent {
 public:
+    // The rail mode for a wheel event specifies the axis on which scrolling is
+    // expected to stick. If this axis is set to Free, then scrolling is not
+    // stuck to any axis.
+    enum RailMode {

[Rick Byers, 2015/03/18 23:30:58]

We'll want to use this from WebGestureEvent too I think.  Perhaps we should move
the definition into WebInputEvent.h?

[ccameron, 2015/03/19 00:05:10]

Good point -- done. Did this to the platform events too.

+        RailModeFree       = 0,
+        RailModeHorizontal = 1 << 0,

[Rick Byers, 2015/03/18 23:30:58]

these don't need to be flags, right?  I.e. it's not legal to specify
'RailModeHorizontal | RailModeVertical', right?

[ccameron, 2015/03/19 00:05:10]

The same is true of Phase, so I was guessing this to be a style thing. Changed
to non-flag-values.

+        RailModeVertical   = 1 << 1,
+    };
+
     enum Phase {
         PhaseNone        = 0,
         PhaseBegan       = 1 << 0,
@@ -390,6 +399,8 @@ public:
     // instead of scroll.
     bool canScroll;
 
+    RailMode railMode;
+
     WebMouseWheelEvent()
         : WebMouseEvent(sizeof(WebMouseWheelEvent))
         , deltaX(0.0f)
@@ -405,6 +416,7 @@ public:
         , scrollByPage(false)
         , hasPreciseScrollingDeltas(false)
         , canScroll(true)
+        , railMode(RailModeFree)
     {
     }
 };