ChannelPosix: Robustly close FDs of deleting messages.
ChannelPosix: Robustly close FDs of deleting messages.

This CL explicitly calls CloseFileDescriptors() to deleting Messages
so that their containing FDs are closed without PCHECK().
Before introducing PlatformFileAttachment, these FDs weren't close()-ed
with PCHECK(), but PlatformFileAttachment closes them with PCHECK(),
and has made closing invalid FDs fatal.

This CL is a speculative crash fix for Mac, just relaxing the check.
It doesn't fix the underlying, long-standing problem.
Ideally we should track down how the FD got invalid.

BUG=504050
R=agl@chromium.org,tsepez@chromium.org

Review URL: https://codereview.chromium.org/1228713008

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#338353}

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