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This CL makes a SingleThreadTaskRunner example more explicit by using
the SingleThreadTaskRunner type instead of using the
SequencedTaskRunner base class.

Bug: N/A
Change-Id: I77fa2affacbda0f4847e4b808eab267f0977fb49
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1636668Reviewed-by: Marijn Kruisselbrink <mek@chromium.org>
Commit-Queue: Dominic Farolino <domfarolino@gmail.com>
Cr-Commit-Position: refs/heads/master@{#665049}