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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.

We found an actual case that JavaScript is executed:
browser_tests --gtest_filter=PrintBrowserTest.SelectionContainsIframe

Bug: 870606
Change-Id: If7027d9a104513408f2d14507f6938308273bd7b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1529501Reviewed-by: Alexander Timin <altimin@chromium.org>
Reviewed-by: Wei Li <weili@chromium.org>
Commit-Queue: Hajime Hoshi <hajimehoshi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#642359}