Add TRACE_EVENT for common events in EventHandler

Often in traces it's hard to know what events are being hanled and how
much time is spent where in the EventDispatcher and EventHandler. This
patch adds a bunch of new TRACE_EVENT's to make it easier.

Review URL: https://codereview.chromium.org/210823007

git-svn-id: svn://svn.chromium.org/blink/trunk@170011 bbb929c8-8fbe-4397-9dbb-9b2b20218538