Update docs for WebResourceRequest.hasUserGesture.

WebResourceRequest.hasUserGesture will be false for new windows for
example. Update docs to say we sometimes say false even though
it 'should' be true.

BUG: 17963383
Change-Id: I877fc7179443647ea6c9dcf8607242874bbe9e09

core/java/android/webkit/WebResourceRequest.java

@@ -42,6 +42,8 @@ public interface WebResourceRequest {
 
     /**
      * Gets whether a gesture (such as a click) was associated with the request.
+     * For security reasons in certain situations this method may return false even though the
+     * sequence of events which caused the request to be created was initiated by a user gesture.
      *
      * @return whether a gesture was associated with the request.
      */