Add devtools trace event for event dispatching

Source/core/events/EventDispatcher.cpp

Index: Source/core/events/EventDispatcher.cpp
diff --git a/Source/core/events/EventDispatcher.cpp b/Source/core/events/EventDispatcher.cpp
index c4485eff66f80cd94cc96649f537921a2e70813a..8f505b55d002f29948fbfbf079110994850b1b7f 100644
--- a/Source/core/events/EventDispatcher.cpp
+++ b/Source/core/events/EventDispatcher.cpp
@@ -33,6 +33,7 @@
 #include "core/events/WindowEventContext.h"
 #include "core/frame/FrameView.h"
 #include "core/inspector/InspectorInstrumentation.h"
+#include "platform/TraceEvent.h"
 #include "wtf/RefPtr.h"
 
 namespace WebCore {
@@ -112,6 +113,8 @@ bool EventDispatcher::dispatch()
     ASSERT(!NoEventDispatchAssertion::isEventDispatchForbidden());
     ASSERT(m_event->target());
     WindowEventContext windowEventContext(m_event.get(), m_node.get(), topNodeEventContext());
+    TRACE_EVENT1(TRACE_DISABLED_BY_DEFAULT("devtools-timeline"), "EventDispatcher::dispatch", "type", m_event->type().ascii());
+    // FIXME(361045): remove InspectorInstrumentation calls once DevTools Timeline migrates to tracing.
     InspectorInstrumentationCookie cookie = InspectorInstrumentation::willDispatchEvent(&m_node->document(), *m_event, windowEventContext.window(), m_node.get(), m_event->eventPath());
 
     void* preDispatchEventHandlerResult;