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..084578ac5b1fe5aa1de82028eab608971e122bd1 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 +1152,14 @@ WebGestureEvent WebInputEventFactory::gestureEvent(NSEvent *event, NSView *view)
         result.type = WebInputEvent::GesturePinchUpdate;
         result.data.pinchUpdate.scale = [event magnification] + 1.0;
         break;
+    case NSEventTypeSmartMagnify:
+        // Map the Cocoa "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 set to 1
+        // because the gesture type already encodes that information.
+        result.type = WebInputEvent::GestureDoubleTap;
+        result.data.tap.tapCount = 1;
+        break;
     case NSEventTypeBeginGesture:
     case NSEventTypeEndGesture:
         // The specific type of a gesture is not defined when the gesture begin