reland 2: Switch back to field trial.

reland 2: "mac: Run A/B experiment on SharedMemory mechanism for resource buffers."

The first reland had two mistakes.
  1) It tried to turn on Mach-based shared memory on 10.6, which isn't supported.
  2) It hit a race condition in AttachmentBrokering code in GPU conformance tests.

I submitted two speculative fixes for (2), but haven't been able to reproduce the problem
locally, so can't verify that the problem is definitively fixed. Please revert this CL if
problems are seen in GPU conformance tests.

> The CL changes the default behavior of resource buffer to use a Mach-backed
> SharedMemory region. This CL adds a field trial to measure the effect of Mach vs
> POSIX backed SharedMemory regions.
>
> BUG=547261, 466437
> Committed: https://crrev.com/e0e2fd398f3d07c8eebbe662d71a0f9286862476
> Cr-Commit-Position: refs/heads/master@{#356513}

BUG=547261, 466437
TBR=mark@chromium.org, isherman@chromium.org, asvitkine@chromium.org, avi@chromium.org

Committed: https://crrev.com/91e69f97803b08f998079bd8ec4729b30e346fbb
Cr-Commit-Position: refs/heads/master@{#357043}

[erikchen, 2015-10-30 00:07:01]

Description was changed from

==========
reland 2: "mac: Run A/B experiment on SharedMemory mechanism for resource
buffers."

The first reland had two mistakes. It tried to turn on Mach-based shared memory
on 10.6, which isn't supported. And it caused flaky errors in the GPU bots.

> The CL changes the default behavior of resource buffer to use a Mach-backed
> SharedMemory region. This CL adds a field trial to measure the effect of Mach
vs
> POSIX backed SharedMemory regions.
>
> BUG=547261, 466437
> Committed: https://crrev.com/e0e2fd398f3d07c8eebbe662d71a0f9286862476
> Cr-Commit-Position: refs/heads/master@{#356513}

BUG=547261, 466437
TBR=mark@chromium.org, isherman@chromium.org, asvitkine@chromium.org,
avi@chromium.org
==========

to

==========
reland 2: "mac: Run A/B experiment on SharedMemory mechanism for resource
buffers."

The first reland had two mistakes. 
  1) It tried to turn on Mach-based shared memory on 10.6, which isn't
supported. 
  2) It hit a race condition in AttachmentBrokering code in GPU conformance
tests.
  
I submitted two speculative fixes for (2), but haven't been able to reproduce
the problem
locally, so can't verify that the problem is definitively fixed. Please revert
this CL if
problems are seen in GPU conformance tests.

> The CL changes the default behavior of resource buffer to use a Mach-backed
> SharedMemory region. This CL adds a field trial to measure the effect of Mach
vs
> POSIX backed SharedMemory regions.
>
> BUG=547261, 466437
> Committed: https://crrev.com/e0e2fd398f3d07c8eebbe662d71a0f9286862476
> Cr-Commit-Position: refs/heads/master@{#356513}

BUG=547261, 466437
TBR=mark@chromium.org, isherman@chromium.org, asvitkine@chromium.org,
avi@chromium.org
==========

[erikchen, 2015-10-30 00:07:05]

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

[commit-bot: I haz the power, 2015-10-30 00:07:36]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1406123010/20001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1406123010/20001

[commit-bot: I haz the power, 2015-10-30 02:04:03]

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

[commit-bot: I haz the power, 2015-10-30 02:04:03]

Dry run: This issue passed the CQ dry run.

[erikchen, 2015-10-30 04:11:27]

The CQ bit was checked by erikchen@chromium.org

[commit-bot: I haz the power, 2015-10-30 04:12:02]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1406123010/20001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1406123010/20001

[commit-bot: I haz the power, 2015-10-30 04:17:04]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2015-10-30 04:18:03]

Patchset 2 (id:??) landed as
https://crrev.com/91e69f97803b08f998079bd8ec4729b30e346fbb
Cr-Commit-Position: refs/heads/master@{#357043}

[erikchen, 2015-10-30 21:02:32]

On 2015/10/30 04:18:03, commit-bot: I haz the power wrote:
> Patchset 2 (id:??) landed as
> https://crrev.com/91e69f97803b08f998079bd8ec4729b30e346fbb
> Cr-Commit-Position: refs/heads/master@{#357043}

Reverting this here: https://codereview.chromium.org/1426663006/

Still causing problems on GPU conformance tests:
http://build.chromium.org/p/chromium.gpu/builders/Mac%20Retina%20Debug/builds...
http://build.chromium.org/p/chromium.gpu/builders/Mac%20Retina%20Debug/builds...