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This simplification is a step towards a lock-free main thread PostTask path.

Split off from: https://chromium-review.googlesource.com/c/chromium/src/+/1304557

Bug: 897751
Change-Id: I6aa2fd98b000b634785f2fee94afa3b11e70423d
Reviewed-on: https://chromium-review.googlesource.com/c/1317568
Commit-Queue: Alex Clarke <alexclarke@chromium.org>
Reviewed-by: Alexander Timin <altimin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#605311}