Fix GenericEventQueue tracing for cancelled events.

Events that were cancelled via clearAllEvents() did not have trace
events associated with them so these events would look like they went
on forever in the tracing visualization. This change provides the
proper trace event to prevent this.

Review URL: https://codereview.chromium.org/45463005

git-svn-id: svn://svn.chromium.org/blink/trunk@161274 bbb929c8-8fbe-4397-9dbb-9b2b20218538