Revert of IPC: Add attachment brokering support to the message header.

Revert of IPC: Add attachment brokering support to the message header. (patchset #3 id:40001 of https://codereview.chromium.org/1303103002/ )

Reason for revert:
Reverting on suspicion of causing crashes in Canary.

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

Original issue's description:
> 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}

TBR=
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=493414
Committed: https://crrev.com/06faf0c0fb1bc6bd173af6152b928e2c80ab8f78
Cr-Commit-Position: refs/heads/master@{#345960}

[erikchen, 2015-08-27 19:48:58]

Created Revert of IPC: Add attachment brokering support to the message header.

[commit-bot: I haz the power, 2015-08-27 19:49:11]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1312433009/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1312433009/1

[commit-bot: I haz the power, 2015-08-27 19:50:04]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-08-27 19:50:39]

Patchset 1 (id:??) landed as
https://crrev.com/06faf0c0fb1bc6bd173af6152b928e2c80ab8f78
Cr-Commit-Position: refs/heads/master@{#345960}