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Currently the async flag in base::File isn't serialized when passing
Files between processes. There are assertions and checks on the value
of this flag in several places, so it needs to be preserved during
serialization/deserialization. On Windows we can't check whether an
existing file handle is async or not, so we have to explicitly pass
the flag around.

Bug: 845612
Change-Id: I8847ecd9451c13f7419e96db465caa417f4c543d
Reviewed-on: https://chromium-review.googlesource.com/1096480
Commit-Queue: Robbie McElrath <rmcelrath@chromium.org>
Reviewed-by: Matt Menke <mmenke@chromium.org>
Reviewed-by: Ken Rockot <rockot@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#568662}