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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

Review URL: https://codereview.chromium.org/663873002

Cr-Commit-Position: refs/heads/master@{#300169}