Implement MessagePumpMojo using WaitSet.

By using a wait set instead of WaitMany(), the message pump
no longer needs to do O(n) iterations over every waiting
handle on every iteration.

BUG=556865

Review URL: https://codereview.chromium.org/1467953002

Cr-Commit-Position: refs/heads/master@{#367716}
4 files changed