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Unlock passed F_UNLCK as cmd to fcntl().  It's supposed to be in
flock.type.  As a result, the file is not actually unlocked.

F_UNLCK is 2 (on OSX and Linux), cmd 2 is F_SETFD, which sets flags on
the file descriptor, which is why the call succeeds.  The only flag
defined appears to be FD_CLOEXEC, which is the low-order bit of the
third parameter.  Since the parameter passed to fcntl is aligned at
off_t or better, Unlock() will clear FD_CLOEXEC if set.  As best I can
tell cases with FD_CLOEXEC do not use base::File.

[Addendum: File::Lock/Unlock was landed for leveldb, and at this time
Chromium's only use is in third_party/leveldatabase/env_chromium.cc.]

BUG=565787

Review URL: https://codereview.chromium.org/1491743009

Cr-Commit-Position: refs/heads/master@{#364497}