DevTools: Instrument GenericEventQueue for async stacks.

This adds async stacks support for the following DOM events:
- audio & video events (play, pause, ratechange, etc.)
- TrackList events: addtrack, removetrack
- MediaSource events (sourceopen and etc.)
- autocomplete and others

BUG=272416
R=yurys, pfeldman@chromium.org

Review URL: https://codereview.chromium.org/335963004

git-svn-id: svn://svn.chromium.org/blink/trunk@176523 bbb929c8-8fbe-4397-9dbb-9b2b20218538