[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-Original-Commit-Position: refs/heads/master@{#603076}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: fcce12fe035224e4574097f3f15398b766276ccd
2 files changed