Add UMA metric to track usage of sending a mousedown to select elements.

third_party/WebKit/Source/core/events/EventDispatcher.cpp

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2001 Dirk Mueller (mueller@kde.org)
  * Copyright (C) 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2011 Apple Inc. All rights reserved.
  * Copyright (C) 2008 Nokia Corporation and/or its subsidiary(-ies)
  * Copyright (C) 2009 Torch Mobile Inc. All rights reserved. (http://www.torchmobile.com/)
  * Copyright (C) 2011 Google Inc. All rights reserved.
  *
  * This library is free software; you can redistribute it and/or
@@ skipping 226 matching lines @@
                 m_event->eventPath()[i].node()->willCallDefaultEventHandler(*m_event);
                 m_event->eventPath()[i].node()->defaultEventHandler(m_event.get());
                 DCHECK(!m_event->defaultPrevented());
                 if (m_event->defaultHandled())
                     break;
             }
         }
         if (m_event->defaultHandled() && !m_event->isTrusted() && !isClick)
             Deprecation::countDeprecation(m_node->document(), UseCounter::UntrustedEventDefaultHandled);
     }
+
+    // Track the usage of sending a mousedown event to a select element to force
+    // it to open. This measures a possible breakage of not allow untrusted

[Rick Byers, 2016/09/01 21:58:56]

nit: s/allow/allowing/

[dtapuska, 2016/09/02 02:57:43]

Done.

+    // events to open select boxes.
+    if (!m_event->isTrusted() && m_event->isMouseEvent() && toMouseEvent(*m_event).type() == EventTypeNames::mousedown && isHTMLSelectElement(*m_node)) {

[Rick Byers, 2016/09/01 21:58:56]

nit: the toMouseEvent is unnecessary, elsewhere in this file we simply do
"m_event->type()".

[dtapuska, 2016/09/02 02:57:43]

Done.

+        UseCounter::count(m_node->document(), UseCounter::UntrustedMouseDownEventDispatched);
+    }
 }
 
 } // namespace blink