Add missing check for quitting from MessagePumpLibevent

If a MessagePumpLibevent has no tasks to run, it will call libevent's
event_base_loop with EVLOOP_ONCE.  However, it may turn out that the
event that finally runs actually quits the MessageLoop.  In this case,
there is an off-by-one error where MessagePumpLibevent will call the
MessageLoop's DoWork function once before realizing that it was actually
asked to quit.  Add a check to prevent this from happening.

BUG=none

Review URL: https://codereview.chromium.org/1019793006

Cr-Commit-Position: refs/heads/master@{#323657}

0 files changed, 0 insertions, 0 deletions