Fix RecvMsgWithPid on broken socket pair

Fix RecvMsgWithPid on broken socket pair

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
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=270967

[jln (very slow on Chromium), 2014-05-08 21:55:04]

lgtm

[mdempsky, 2014-05-08 21:56:26]

ajwong: Will you please review and approve for base OWNERS?  Thanks!

[mdempsky, 2014-05-12 17:02:28]

On 2014/05/08 21:56:26, mdempsky wrote:
> ajwong: Will you please review and approve for base OWNERS?  Thanks!

Ping?  Small DCHECK logic fix, includes a test case, and already reviewed by
Julien. :)

[awong, 2014-05-16 05:46:34]

LGTM

sorry for the delay

[mdempsky, 2014-05-16 05:50:20]

The CQ bit was checked by mdempsky@chromium.org

[commit-bot: I haz the power, 2014-05-16 05:50:50]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/mdempsky@chromium.org/276593008/20001

[commit-bot: I haz the power, 2014-05-16 07:51:51]

FYI, CQ is re-trying this CL (attempt #1).
Please consider checking whether the failures are real,
and report flakes to chrome-troopers@google.com.
The failing builders are:
  android_dbg on tryserver.chromium
(http://build.chromium.org/p/tryserver.chromium/builders/android_dbg/builds/18...)

[commit-bot: I haz the power, 2014-05-16 08:57:59]

Change committed as 270967