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This component provides one dedicated thread per
SingleThreadTaskRunner.

Reference Change:
https://codereview.chromium.org/2686593003/

BUG=684080, 694823, 697697

Review-Url: https://codereview.chromium.org/2698843006
Cr-Original-Commit-Position: refs/heads/master@{#453162}
Committed: https://chromium.googlesource.com/chromium/src/+/fc203f52e046a8d82e053525d764671f7128f3d4
Review-Url: https://codereview.chromium.org/2698843006
Cr-Commit-Position: refs/heads/master@{#456148}