IPC: Add attachment brokering support to the message header.

Reland #1: IPC: Add attachment brokering support to the message header.

This reland fixes a race condition in the unit test SendHandleTwice that caused
the test to flakily fail, mostly on XP machines. This reland also updates switch
statements to contain a block for the newly added enum
BrokerableAttachment::PLACEHOLDER, which was causing problems with the clang
Windows build.

> Message dispatch happens before message translation, and message dispatch
> requires that all brokered attachments have been received. This means that
> attachment brokering needs to function without message translation. This is
> accomplished by modifying the message header to include a new field
> num_brokered_attachments, and writing the attachment ids into the IPC Channel
> immediately following the pickled message itself.
>
> AttachmentBrokerPrivilegedWinUnittest was expanded to test ChannelReader in the
> receiving process. It is now a fully functional end-to-end test of attachment
> brokering.
>
> BUG=493414
TBR=tsepez@chromium.org
BUG=493414
Committed: https://crrev.com/37a2e0b682555bf35852d707dbd74b68f345841f
Cr-Commit-Position: refs/heads/master@{#344933}

[erikchen, 2015-08-21 22:58:37]

The CQ bit was checked by erikchen@chromium.org

[commit-bot: I haz the power, 2015-08-21 22:59:30]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1303103002/40001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1303103002/40001

[commit-bot: I haz the power, 2015-08-21 23:21:05]

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

[commit-bot: I haz the power, 2015-08-21 23:21:06]

Try jobs failed on following builders:
  android_arm64_dbg_recipe on tryserver.chromium.linux (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.linux/builders/android_arm64_d...)
  android_compile_dbg on tryserver.chromium.linux (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.linux/builders/android_compile...)
  linux_android_rel_ng on tryserver.chromium.linux (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.linux/builders/linux_android_r...)

[erikchen, 2015-08-21 23:33:15]

The CQ bit was checked by erikchen@chromium.org

[commit-bot: I haz the power, 2015-08-21 23:34:48]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1303103002/40001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1303103002/40001

[commit-bot: I haz the power, 2015-08-22 00:26:43]

Committed patchset #3 (id:40001)

[commit-bot: I haz the power, 2015-08-22 00:27:36]

Patchset 3 (id:??) landed as
https://crrev.com/37a2e0b682555bf35852d707dbd74b68f345841f
Cr-Commit-Position: refs/heads/master@{#344933}

[erikchen, 2015-08-27 19:48:57]

A revert of this CL (patchset #3 id:40001) has been created in
https://codereview.chromium.org/1312433009/ by erikchen@chromium.org.

The reason for reverting is: Reverting on suspicion of causing crashes in
Canary.

https://code.google.com/p/chromium/issues/detail?id=524032.