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

This reverts commit fcce12fe035224e4574097f3f15398b766276ccd.

Reason for revert: base_unittests failures: crbug.com/899263.

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: François Doray <fdoray@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#603076}

TBR=gab@chromium.org,fdoray@chromium.org,altimin@chromium.org

Change-Id: I1185c518e8243837cafdbee68d163b0cfdf611fc
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 891670
Reviewed-on: https://chromium-review.googlesource.com/c/1302013
Reviewed-by: Alexander Timin <altimin@chromium.org>
Commit-Queue: Alexander Timin <altimin@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#603115}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 17956924e472a949e04668b18de0c0a93b0bc4be
2 files changed