Deprecate invoking the default action for untrusted events.

third_party/WebKit/Source/core/frame/Deprecation.cpp

 // 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.
 
 #include "core/frame/Deprecation.h"
 
 #include "core/dom/Document.h"
 #include "core/dom/ExecutionContext.h"
 #include "core/frame/FrameConsole.h"
 #include "core/frame/FrameHost.h"
@@ skipping 370 matching lines @@
 
     case UseCounter::During_Microtask_Prompt:
         return willBeRemoved("Invoking 'prompt()' during microtask execution", 53, "5647113010544640");
 
     case UseCounter::During_Microtask_SyncXHR:
         return willBeRemoved("Invoking 'send()' on a sync XHR during microtask execution", 53, "5647113010544640");
 
     case UseCounter::MediaStreamOnEnded:
         return replacedWillBeRemoved("The MediaStream 'ended' event", "the MediaStream 'inactive' event", 53, "5730404371791872");
 
+    case UseCounter::UntrustedEventDefaultHandled:
+        return String::format("A DOM event generated from JavaScript has triggered a default action inside the browser. This behavior is non-standard and will be removed in %s. See https://www.chromestatus.com/features/5718803933560832 for more details.", milestoneString(53));
+
     // Features that aren't deprecated don't have a deprecation message.
     default:
         return String();
     }
 }
 
 } // namespace blink