[Android] Always precede Tap gesture events with TapDown

content/common/input/gesture_event_stream_validator.cc

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "content/common/input/gesture_event_stream_validator.h"
 
 #include "base/logging.h"
 #include "third_party/WebKit/public/web/WebInputEvent.h"
 
 using blink::WebInputEvent;
@@ skipping 54 matching lines @@
     case WebInputEvent::GestureTapUnconfirmed:
       if (!waiting_for_tap_end_)
         error_msg->append("Missing TapDown event before TapUnconfirmed\n");
       break;
     case WebInputEvent::GestureTapCancel:
       if (!waiting_for_tap_end_)
         error_msg->append("Missing TapDown event before TapCancel\n");
       waiting_for_tap_end_ = false;
       break;
     case WebInputEvent::GestureTap:
+      if (!waiting_for_tap_end_)
+        error_msg->append("Missing TapDown event before Tap\n");
+      waiting_for_tap_end_ = false;
+      break;
     case WebInputEvent::GestureDoubleTap:
-      // Both Tap and DoubleTap gestures may be synthetically inserted, and do
-      // not require a preceding TapDown.
+      // DoubleTap gestures may be synthetically inserted, and do not require a
+      // preceding TapDown.
       waiting_for_tap_end_ = false;
       break;
     default:
       break;
   }
   return error_msg->empty();
 }
 
 }  // namespace content