Add rails to input wheel events.

Source/platform/PlatformWheelEvent.h

Index: Source/platform/PlatformWheelEvent.h
diff --git a/Source/platform/PlatformWheelEvent.h b/Source/platform/PlatformWheelEvent.h
index fc36ffb137765dacc4348ade05fd8f90438d039c..b282da1437baac9dc0a53b416a60f51afd2279c4 100644
--- a/Source/platform/PlatformWheelEvent.h
+++ b/Source/platform/PlatformWheelEvent.h
@@ -41,6 +41,12 @@ enum PlatformWheelEventGranularity {
     ScrollByPixelWheelEvent,
 };
 
+enum PlatformWheelEventRailMode {
+    PlatformWheelEventRailModeFree       = 0,
+    PlatformWheelEventRailModeHorizontal = 1 << 0,
+    PlatformWheelEventRailModeVertical   = 1 << 1,
+};
+
 #if OS(MACOSX)
 enum PlatformWheelEventPhase {
     PlatformWheelEventPhaseNone        = 0,
@@ -64,6 +70,7 @@ public:
         , m_granularity(ScrollByPixelWheelEvent)
         , m_hasPreciseScrollingDeltas(false)
         , m_canScroll(true)
+        , m_railMode(PlatformWheelEventRailModeFree)
 #if OS(MACOSX)
         , m_phase(PlatformWheelEventPhaseNone)
         , m_momentumPhase(PlatformWheelEventPhaseNone)
@@ -84,6 +91,7 @@ public:
         , m_granularity(granularity)
         , m_hasPreciseScrollingDeltas(false)
         , m_canScroll(true)
+        , m_railMode(PlatformWheelEventRailModeFree)
 #if OS(MACOSX)
         , m_phase(PlatformWheelEventPhaseNone)
         , m_momentumPhase(PlatformWheelEventPhaseNone)
@@ -108,6 +116,8 @@ public:
     void setHasPreciseScrollingDeltas(bool b) { m_hasPreciseScrollingDeltas = b; }
     bool canScroll() const { return m_canScroll; }
     void setCanScroll(bool b) { m_canScroll = b; }
+    PlatformWheelEventRailMode railMode() const { return m_railMode; }
+
 #if OS(MACOSX)
     PlatformWheelEventPhase phase() const { return m_phase; }
     PlatformWheelEventPhase momentumPhase() const { return m_momentumPhase; }
@@ -128,6 +138,7 @@ protected:
     PlatformWheelEventGranularity m_granularity;
     bool m_hasPreciseScrollingDeltas;
     bool m_canScroll;
+    PlatformWheelEventRailMode m_railMode;
 #if OS(MACOSX)
     PlatformWheelEventPhase m_phase;
     PlatformWheelEventPhase m_momentumPhase;