Reland "[base] Use TaskRunner instead of MessageLoop in FileDescriptorWatcher"

This is a reland of fcce12fe035224e4574097f3f15398b766276ccd

TBR=gab@chromium.org
BUG=891670

Original change's description:
> [base] Use TaskRunner instead of MessageLoop in FileDescriptorWatcher
>
> Do not use MessageLoopForIO as it is used to post tasks anyway.
>
> R=gab@chromium.org
> TBR=gab@chromium.org
> BUG=891670
>
> Change-Id: Ib255c6a5a5dd55c2654eaa284d43d7f78f06b244
> Reviewed-on: https://chromium-review.googlesource.com/c/1299239
> Commit-Queue: Alexander Timin <altimin@chromium.org>
> Reviewed-by: Alexander Timin <altimin@chromium.org>
> Reviewed-by: Gabriel Charette <gab@chromium.org>
> Reviewed-by: Fran├žois Doray <fdoray@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#603076}

Bug: 891670
Change-Id: I80392385f1b63226d819e490132b6c4f8cf5198c
Reviewed-on: https://chromium-review.googlesource.com/c/1303714
Reviewed-by: Alexander Timin <altimin@chromium.org>
Commit-Queue: Alexander Timin <altimin@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#603338}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 6247f6a91ade61d198c618966cdb2669beab981b
2 files changed