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This CL switches task runners at several places from per-thread to
per-frame. The purpose of this change is to make it possible to freeze
tasks for bfcache.

bfcache is a new feature to freeze a tab for quick back-forwarding. When
a tab goes frozen, we can stop per-frame task runners while we cannot
per-thread task runners since per-thread task runners can be shared
between multiple tags/pages. As a frozen tab must not execute any
JavaScript, all JavaScript execution must be on per-frame task runners,
which can be suspended.

Bug: 913912
Change-Id: I5dcf4d2fa650469fda473cae0b5a1e2d603176ca
Reviewed-on: https://chromium-review.googlesource.com/c/1438855
Commit-Queue: Hajime Hoshi <hajimehoshi@chromium.org>
Reviewed-by: Alexander Timin <altimin@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#626902}