Add missing check for quitting from MessagePumpLibevent

Re-land: 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=473693
Committed: https://crrev.com/ee9b754ba442a4c77c67f56caf465b3d9d13d97f
Cr-Commit-Position: refs/heads/master@{#323834}

[Chirantan Ekbote, 2015-04-03 20:26:17]

chirantan@chromium.org changed reviewers:
+ sadrul@chromium.org, thestig@chromium.org

[Chirantan Ekbote, 2015-04-03 20:26:17]

ptal

[Lei Zhang, 2015-04-03 21:45:05]

lgtm

[Chirantan Ekbote, 2015-04-03 21:52:52]

The CQ bit was checked by chirantan@chromium.org

[commit-bot: I haz the power, 2015-04-03 21:53:25]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1059353003/20001

[commit-bot: I haz the power, 2015-04-03 22:28:26]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2015-04-03 22:29:50]

Patchset 2 (id:??) landed as
https://crrev.com/ee9b754ba442a4c77c67f56caf465b3d9d13d97f
Cr-Commit-Position: refs/heads/master@{#323834}