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Second step of a a three sided patch to move IdleTask to WebThread.
This is needed because we plan to introduce a scheduler for worker
threads, and they will use the WebTherad interface for posting idle
tasks.

Other patches https://codereview.chromium.org/1031843002/
and https://codereview.chromium.org/1029803002

BUG=463143

Review URL: https://codereview.chromium.org/1029343002

Cr-Commit-Position: refs/heads/master@{#322443}