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

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

Index: third_party/WebKit/Source/core/events/EventDispatcher.cpp
diff --git a/third_party/WebKit/Source/core/events/EventDispatcher.cpp b/third_party/WebKit/Source/core/events/EventDispatcher.cpp
index a20c4a0a1a4dfcd1791781c965ae3fc5155f92a4..284422958625c02a0b5452bc7faa68e154ac542e 100644
--- a/third_party/WebKit/Source/core/events/EventDispatcher.cpp
+++ b/third_party/WebKit/Source/core/events/EventDispatcher.cpp
@@ -244,6 +244,13 @@ inline void EventDispatcher::dispatchEventPostProcess(EventDispatchHandlingState
         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