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PipeMessagingChannel takes ownership of the passed File objects,
closing them. This means that stdin/stdout are closed, to prevent
accidental corruption of the native-messaging streams.

This CL reopens stdin/stdout against /dev/null, which protects them,
preventing any future calls to POSIX open() returning these descriptors.
For example, LaunchProcess (by default) treats FDs 0, 1, 2 specially
(it closes all descriptors except them), so having stdin/stdout directed
to some random file could cause unintended effects.

This CL also fixes PipeMessagingChannel ctor to not interleave the dup()
and close() calls:
dup, dup, close, close
instead of
dup, close, dup, close
so that the second dup() call now returns a high-numbered descriptor
instead of 0 (freed up by first close() call).

Bug: 1029858
Change-Id: I209f5734a06216323c54a680852b381fb6f2f96b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1947397
Commit-Queue: Lambros Lambrou <lambroslambrou@chromium.org>
Reviewed-by: Jamie Walch <jamiewalch@chromium.org>
Auto-Submit: Lambros Lambrou <lambroslambrou@chromium.org>
Cr-Commit-Position: refs/heads/master@{#720768}