Update content module for IPC attachment brokering.

Update content module for IPC attachment brokering.

This CL has no functional effect, although it stubs in necessary placeholders
for IPC attachment brokering.

The browser process now has access to a singleton AttachmentBroker* via
content::ChildProcessHost::GetAttachmentBroker().

Child processes now have access to an AttachmentBroker* via
content::ChildThread. Each instance of ChildThread creates an AttachmentBroker
during Init().

BUG=493414
Committed: https://crrev.com/f7f98882ff232dd35cb03c76b78a8b51bd6643a0
Cr-Commit-Position: refs/heads/master@{#334958}

[erikchen, 2015-06-16 18:17:39]

The CQ bit was checked by erikchen@chromium.org to run a CQ dry run

[commit-bot: I haz the power, 2015-06-16 18:18:16]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1187153003/1

[erikchen, 2015-06-16 20:06:30]

The CQ bit was checked by erikchen@chromium.org to run a CQ dry run

[commit-bot: I haz the power, 2015-06-16 20:08:27]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1187153003/60001

[erikchen, 2015-06-16 20:12:39]

The CQ bit was checked by erikchen@chromium.org to run a CQ dry run

[commit-bot: I haz the power, 2015-06-16 20:14:46]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1187153003/70001

[erikchen, 2015-06-16 20:35:36]

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

[erikchen, 2015-06-16 20:35:37]

tsepez: Please review.

I will be sending to avi@ afterwards for a content OWNER review.

[Tom Sepez, 2015-06-16 21:18:33]

On 2015/06/16 20:35:37, erikchen wrote:
> tsepez: Please review.
> 
> I will be sending to avi@ afterwards for a content OWNER review.

LGTM.

[erikchen, 2015-06-16 21:18:56]

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

[erikchen, 2015-06-16 21:18:57]

avi: Looking for an OWNER review.

[erikchen, 2015-06-16 23:50:14]

The CQ bit was checked by erikchen@chromium.org to run a CQ dry run

[commit-bot: I haz the power, 2015-06-16 23:51:14]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1187153003/90001

[commit-bot: I haz the power, 2015-06-16 23:56:15]

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

[commit-bot: I haz the power, 2015-06-16 23:56:16]

Dry run: Try jobs failed on following builders:
  ios_dbg_simulator_ninja on tryserver.chromium.mac (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.mac/builders/ios_dbg_simulator...)
  ios_rel_device_ninja on tryserver.chromium.mac (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.mac/builders/ios_rel_device_ni...)
  mac_chromium_compile_dbg_ng on tryserver.chromium.mac (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.mac/builders/mac_chromium_comp...)
  mac_chromium_rel_ng on tryserver.chromium.mac (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.mac/builders/mac_chromium_rel_...)

[erikchen, 2015-06-17 00:03:22]

The CQ bit was checked by erikchen@chromium.org to run a CQ dry run

[erikchen, 2015-06-17 00:03:22]

The patchset sent to the CQ was uploaded after l-g-t-m from tsepez@chromium.org
Link to the patchset: https://codereview.chromium.org/1187153003/#ps110001
(title: "Rebase against top of tree. (plugin_service_browsertest was removed)")

[commit-bot: I haz the power, 2015-06-17 00:04:53]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1187153003/110001

[commit-bot: I haz the power, 2015-06-17 01:42:03]

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

[commit-bot: I haz the power, 2015-06-17 01:42:05]

Dry run: This issue passed the CQ dry run.

[Avi (use Gerrit), 2015-06-17 15:34:58]

I'm a bit lost as to the context here, and given that one of the rules of
content/ is to keep chrome/ abstractions out, I'm a bit surprised to see in a
comment the literal statement of "we're doing this one thing in content/ because
of chrome/".

[Avi (use Gerrit), 2015-06-17 20:32:12]

As discussed in chat, this was a misunderstanding over a comment. LGTM

[erikchen, 2015-06-17 20:34:41]

The CQ bit was checked by erikchen@chromium.org

[erikchen, 2015-06-17 20:34:41]

The patchset sent to the CQ was uploaded after l-g-t-m from tsepez@chromium.org,
avi@chromium.org
Link to the patchset: https://codereview.chromium.org/1187153003/#ps150001
(title: "Use normal inheritance for SupportsAttachmentBrokering rather than
virtual inheritance.")

[commit-bot: I haz the power, 2015-06-17 20:37:30]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1187153003/150001

[commit-bot: I haz the power, 2015-06-17 20:56:41]

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

[commit-bot: I haz the power, 2015-06-17 20:56:43]

Try jobs failed on following builders:
  android_chromium_gn_compile_dbg on tryserver.chromium.linux (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.linux/builders/android_chromiu...)
  linux_chromium_gn_rel on tryserver.chromium.linux (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.linux/builders/linux_chromium_...)

[erikchen, 2015-06-17 21:13:11]

The CQ bit was checked by erikchen@chromium.org

[erikchen, 2015-06-17 21:13:11]

The patchset sent to the CQ was uploaded after l-g-t-m from tsepez@chromium.org,
avi@chromium.org
Link to the patchset: https://codereview.chromium.org/1187153003/#ps170001
(title: "Add virtual inheritance back to ChildThread, since it's involved in a
diamond inheritance hierarchy.")

[commit-bot: I haz the power, 2015-06-17 21:14:52]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1187153003/170001

[commit-bot: I haz the power, 2015-06-17 21:34:15]

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

[commit-bot: I haz the power, 2015-06-17 21:34:18]

Try jobs failed on following builders:
  android_chromium_gn_compile_rel on tryserver.chromium.linux (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.linux/builders/android_chromiu...)
  linux_chromium_gn_dbg on tryserver.chromium.linux (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.linux/builders/linux_chromium_...)
  linux_chromium_gn_rel on tryserver.chromium.linux (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.linux/builders/linux_chromium_...)

[erikchen, 2015-06-17 21:44:34]

The CQ bit was checked by erikchen@chromium.org

[erikchen, 2015-06-17 21:44:35]

The patchset sent to the CQ was uploaded after l-g-t-m from tsepez@chromium.org,
avi@chromium.org
Link to the patchset: https://codereview.chromium.org/1187153003/#ps190001
(title: "GPUChannelHostFactory also needs to virtually inherit from
SupportsAttachmentBrokering. This patch …")

[commit-bot: I haz the power, 2015-06-17 21:46:55]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1187153003/190001

[commit-bot: I haz the power, 2015-06-17 23:27:16]

Committed patchset #11 (id:190001)

[commit-bot: I haz the power, 2015-06-17 23:28:57]

Patchset 11 (id:??) landed as
https://crrev.com/f7f98882ff232dd35cb03c76b78a8b51bd6643a0
Cr-Commit-Position: refs/heads/master@{#334958}