Mac: Enable two finger double-tap smart zoom

Source/web/WebInputEventFactoryMac.mm

Index: Source/web/WebInputEventFactoryMac.mm
diff --git a/Source/web/WebInputEventFactoryMac.mm b/Source/web/WebInputEventFactoryMac.mm
index 572b3e25d8976378694c7cc280baf6854805645f..f2ec180204237cecaf6841d96e3a6fe23f68b32a 100644
--- a/Source/web/WebInputEventFactoryMac.mm
+++ b/Source/web/WebInputEventFactoryMac.mm
@@ -62,6 +62,10 @@ enum {
     NSEventPhaseMayBegin    = 0x1 << 5
 };
 
+enum {
+    NSEventTypeSmartMagnify = 32
+};
+
 #endif  // __MAC_OS_X_VERSION_MAX_ALLOWED < 1080
 
 namespace blink {
@@ -1148,6 +1152,10 @@ WebGestureEvent WebInputEventFactory::gestureEvent(NSEvent *event, NSView *view)
         result.type = WebInputEvent::GesturePinchUpdate;
         result.data.pinchUpdate.scale = [event magnification] + 1.0;
         break;
+    case NSEventTypeSmartMagnify:
+        result.type = WebInputEvent::GestureDoubleTap;

[aelias_OOO_until_Jul13, 2015/02/07 01:48:00]

I think this is worth a comment:

// Map the OS X "double-tap with two fingers" zoom gesture to regular
GestureDoubleTap, because the effect is similar to single-finger double-tap zoom
on mobile platforms.  Note that tapCount is intentionally 1 since the gesture
type already encodes the information.

[ccameron, 2015/02/07 08:19:41]

Good idea (and thanks for the wording suggestion).

+        result.data.tap.tapCount = 1;
+        break;
     case NSEventTypeBeginGesture:
     case NSEventTypeEndGesture:
         // The specific type of a gesture is not defined when the gesture begin