Add the ability to observe MessagePumpMojo

Add the ability to observe MessagePumpMojo

We need the ability to observe the Mojo message pump in order to correctly
resolve promises at the end of microtasks triggered by signaling Mojo handles.

Another approach James and I discussed for this CL was to trigger
MessageLoop::TaskObserver, but that requires a PendingTask, which we don't
have. I investigated removing the PendingTask argument from
MessageLoop::TaskObserver, but it's used in a number of tests. Instead, this CL
follows the approach used by MessagePumpLibevent and introduces a specialized
observer interface for the message pump.

R=jamesr@chromium.org

Committed: https://crrev.com/4ba3cada588332b5e67b326c35dec8a0cc779d45
Cr-Commit-Position: refs/heads/master@{#300169}

[jamesr, 2014-10-17 19:35:37]

lgtm

[abarth-chromium, 2014-10-17 20:57:30]

The CQ bit was checked by abarth@chromium.org

[commit-bot: I haz the power, 2014-10-17 20:58:55]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/663873002/1

[commit-bot: I haz the power, 2014-10-17 22:02:38]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2014-10-17 22:04:24]

Patchset 1 (id:??) landed as
https://crrev.com/4ba3cada588332b5e67b326c35dec8a0cc779d45
Cr-Commit-Position: refs/heads/master@{#300169}