ipc: Add a member to IPC::Message::Header on Windows.

ipc: Add a member to IPC::Message::Header on Windows.

According to my most recent debugging experiments, the root cause of the crashes
that happened the first time I tried to make this change have magically
disappeared. See the bug for more details.

BUG=527588
Committed: https://crrev.com/c32bea67c022c6c385463f0eec027f18b2915300
Cr-Commit-Position: refs/heads/master@{#354889}

[erikchen, 2015-10-08 23:44:10]

erikchen@chromium.org changed reviewers:
+ tsepez@chromium.org

[erikchen, 2015-10-08 23:44:10]

tsepez: Please review.

[Tom Sepez, 2015-10-09 17:05:09]

lgtm

[erikchen, 2015-10-09 17:31:57]

The CQ bit was checked by erikchen@chromium.org

[commit-bot: I haz the power, 2015-10-09 17:33:10]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1400713002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1400713002/1

[commit-bot: I haz the power, 2015-10-09 18:14:27]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2015-10-09 18:14:28]

Try jobs failed on following builders:
  win_chromium_compile_dbg_ng on tryserver.chromium.win (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.win/builders/win_chromium_comp...)

[erikchen, 2015-10-19 21:13:00]

The CQ bit was checked by erikchen@chromium.org

[commit-bot: I haz the power, 2015-10-19 21:14:10]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1400713002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1400713002/1

[commit-bot: I haz the power, 2015-10-19 22:23:39]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-10-19 22:24:50]

Patchset 1 (id:??) landed as
https://crrev.com/c32bea67c022c6c385463f0eec027f18b2915300
Cr-Commit-Position: refs/heads/master@{#354889}

[erikchen, 2015-10-20 17:21:22]

A revert of this CL (patchset #1 id:1) has been created in
https://codereview.chromium.org/1414293002/ by erikchen@chromium.org.

The reason for reverting is: Likely causes crashes:
https://code.google.com/p/chromium/issues/detail?id=527588#c57.