Refactor mobile browser detection to be CSP-compatible.

The previous code generates CSP (Content-Security-Policy) violations on
mobile devices when CSP is enabled. The new code should be equivalent
in functionality. Manually tested on Chrome 61 on a Nexus 6p.

Change-Id: Id5838b68352516cd453284aa022ef69adee80c06
Review-Link: https://gwt-review.googlesource.com/#/c/19620/
diff --git a/user/src/com/google/gwt/event/dom/client/TouchEvent.java b/user/src/com/google/gwt/event/dom/client/TouchEvent.java
index 556c3cb..e4fc827 100644
--- a/user/src/com/google/gwt/event/dom/client/TouchEvent.java
+++ b/user/src/com/google/gwt/event/dom/client/TouchEvent.java
@@ -41,9 +41,7 @@
     }
 
     private native boolean detectTouchSupport() /*-{
-      var elem = document.createElement('div');
-      elem.setAttribute('ontouchstart', 'return;');
-      return (typeof elem.ontouchstart) == "function";
+      return 'ontouchstart' in $wnd;
     }-*/;
   }