Make Mac pinch thresholding apply only to zoom (Blink side)

public/web/WebInputEvent.h

Index: public/web/WebInputEvent.h
diff --git a/public/web/WebInputEvent.h b/public/web/WebInputEvent.h
index 85a576a3a95b8b182cfec4a7702f2766a79de9bc..6945f4ce63aaace89dd1b63b538312ac1c13e04e 100644
--- a/public/web/WebInputEvent.h
+++ b/public/web/WebInputEvent.h
@@ -498,6 +498,7 @@ public:
         } flingCancel;
 
         struct {
+            bool zoomDisabled;

[Rick Byers, 2015/05/06 21:53:04]

nit: can we call this canZoom since it almost exactly parallels the
WebMouseWheelEvent::canScroll bit?  Also please add a comment saying why we need
this bit.

[ccameron, 2015/05/06 22:10:14]

I went with zoomDisabled so that the default value (the constructor memsets the
struct to 0) would match the default behavior. I can change it to canZoom, but
if I do, I'd like to update the constructor to initialize it to true then --
sound reasonable?

             float scale;
         } pinchUpdate;
     } data;