Update WebInputEventFactory on Mac for pinch gesture support

WebInputEventFactory::gestureEvent was added a long time ago but is
not currently used.  Remove the existing unused code and update the
function to support creating GesturePinchUpdate events from Cocoa
NSEventTypeMagnify events.

BUG=289887

Review URL: https://codereview.chromium.org/184983003

git-svn-id: svn://svn.chromium.org/blink/trunk@168482 bbb929c8-8fbe-4397-9dbb-9b2b20218538