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This prevents RecvMsgWithPid from DCHECK failing under Debug builds
when reading EOF from a broken socket pair.  Noticed while testing
crrev.com/269543014.

Unfortunately, Linux allows sending zero length messages that the
recvmsg() API does not distinguish from reading EOF.  To be
conservative, if we didn't receive any message or control bytes, then
assume we received an EOF and tolerate not knowing the sender's PID.

BUG=357670

Review URL: https://codereview.chromium.org/276593008

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@270967 0039d316-1c4b-4281-b951-d872f2087c98