GitLab

This change will give WebRTC the benefits of Chromium's checks and integration with thread and sequence runners (e.g. io and waitable checks).

Followup changes include enabling the code from the WebRTC side (requires build file changes) and integrating the TaskQueue implementation in Chromium with sequence runners.

BUG=689520
NOTRY=true

Review-Url: https://codereview.chromium.org/2975553002
Cr-Commit-Position: refs/heads/master@{#486737}