Adding browser side Worker handling.
This tracks Worker construction, events, and actions.
Making WTF run in workers.
Retry injection if a failure occurs.
Rewriting DOM provider/adding XHR provider/Worker provider.
This moves a lot of the shared EventTarget emulation/proxying code
to wtf.trace.eventtarget.
Worker snapshotting is not yet implemented as it requires a redesign of
the snapshot API.
Fixing document re-open.
Cleaning up the proxy property generation code.
Typo fix.
Merge pull request #188 from google/workers

Refactoring DOM injection and adding better tracing of workers and XHR.