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This commit simplifies the logic and puts only the blocking code in the
created USER_VISIBLE sequenced task runner. In this way, most of the
tasks are run on the UI thread.

BUG=b:175355860
TEST=run on octopus

Change-Id: I50b9380c5d0667513430f4f3b685317687be844d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2586288
Commit-Queue: Cheng-Hao Yang <chenghaoyang@chromium.org>
Reviewed-by: Ahmed Fakhry <afakhry@chromium.org>
Cr-Commit-Position: refs/heads/master@{#839057}