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This CL moves the definition of MessageLoopCurrent::TaskObserver to
an individual header task/task_observer.h.

The motivation of this change comes from crrev.com/c/1295617.
MessageLoop is going away soon(-ish?), and Blink doesn't want to depend
on base::MessageLoop. It makes more sense to have TaskObserver in a
stand-alone header, given this current situation.

This CL does not touch any use sites of TaskObserver. The alias
MessageLoop::TaskObserver is still available, pending future mass
replacements.

Bug: 826203
Change-Id: I9e17bae3586e29cb76b23f5e247307760dac9a1f
Reviewed-on: https://chromium-review.googlesource.com/c/1298944
Commit-Queue: Yuta Kitamura <yutak@chromium.org>
Reviewed-by: Alexander Timin <altimin@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#603015}