Plumb gesture source value through Blink.

third_party/WebKit/public/platform/WebGestureDevice.h

 /*
  * Copyright (C) 2014 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  *
  * 1.  Redistributions of source code must retain the above copyright
  *     notice, this list of conditions and the following disclaimer.
  * 2.  Redistributions in binary form must reproduce the above copyright
@@ skipping 11 matching lines @@
  * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
  * THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */
 
 #ifndef WebGestureDevice_h
 #define WebGestureDevice_h
 
 namespace blink {
 
 enum WebGestureDevice {
+    WebGestureDeviceUninitialized,
     WebGestureDeviceTouchpad,
     WebGestureDeviceTouchscreen,
 };
 
 } // namespace blink
 
 #endif // WebGestureDevice_h