Make Thread::TaskObserver an alias to base::MessageLoop's.

This is a simple refactoring that changes the type of the task observer
used in Blink. We still keep the type alias Thread::TaskObserver,
as we don't want to expose base::MessageLoop to Blink.

This CL also updates audit_non_blink_usage.py to allow base::
PendingTask in Blink.

Bug: 826203
Change-Id: I6ac75469abf92d171628dbaa14c330a344129f8b
Reviewed-on: https://chromium-review.googlesource.com/c/1295617
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Alexander Timin <altimin@chromium.org>
Commit-Queue: Yuta Kitamura <yutak@chromium.org>
Cr-Commit-Position: refs/heads/master@{#602663}
22 files changed