Add devtools trace event for event dispatching

Current implementation of InspectorTimelineAgent will emit EventDispatch event only if there is at least one handler on the event's path. After this patch EventDispather::dispatch trace events will be sent for unhandled events too. We should be able to filter out such events later on the front-end side and avoid additional checks at the instrumentation point.

BUG=361045

Review URL: https://codereview.chromium.org/239303003

git-svn-id: svn://svn.chromium.org/blink/trunk@171951 bbb929c8-8fbe-4397-9dbb-9b2b20218538