Enumerate the return value of dispatchEvent so it is clear.

third_party/WebKit/Source/core/events/EventDispatchResult.h

+// Copyright 2016 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.
+
+#ifndef EventDispatchResult_h
+#define EventDispatchResult_h
+
+namespace blink {
+
+enum class DispatchEventResult {
+    // Event was not canceled by event handler or default event handler.
+    NotCanceled,
+    // Event was canceled by event handler; i.e. a script handler calling preventDefault.
+    CanceledByEventHandler,
+    // Event was canceled by the default event handler; i.e. executing the default action.
+    // This result should be used sparingly as it deviates from the DOM Event Dispatch
+    // model. Default event handlers really shouldn't be invoked inside of dispatch.
+    CanceledByDefaultEventHandler,
+    // Event was canceled but suppressed before dispatched to event handler.
+    // This result should be used sparingly; and its usage likely indicates
+    // there is potential for a bug. Trusted events may return this code; but untrusted
+    // events likely should always execute the event handler the developer intends to
+    // execute.
+    CanceledBeforeDispatch,
+};
+
+} // namespace blink
+
+#endif // EventDispatchResult_h