Revert of base: Make shared memory backed by Mach primitives by default.

Revert of base: Make shared memory backed by Mach primitives by default. (patchset #3 id:40001 of https://codereview.chromium.org/1453383002/ )

Reason for revert:
Speculative revert: 3 mac perf bots started timing out in random tests around the time this patch landed. I'll keep an eye on the waterfall and reland if things don't improve.

See https://code.google.com/p/chromium/issues/detail?id=561416 for logs.

Original issue's description:
> base: Make shared memory backed by Mach primitives by default.
>
> The type of shared memory used is controlled by an A/B experiment. If the
> experiment isn't present, use Mach primitives.
>
> BUG=466437
>
> Committed: https://crrev.com/8b8e4212cef52b2cf924c27bb7c49e375306acb3
> Cr-Commit-Position: refs/heads/master@{#361249}

TBR=mark@chromium.org,avi@chromium.org,erikchen@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=466437
Committed: https://crrev.com/3e6b742e634e657d87c3267c2a8f9158c297f801
Cr-Commit-Position: refs/heads/master@{#361630}

[Sami, 2015-11-25 12:12:58]

Created Revert of base: Make shared memory backed by Mach primitives by default.

[commit-bot: I haz the power, 2015-11-25 12:13:25]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1472313002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1472313002/1

[commit-bot: I haz the power, 2015-11-25 12:14:29]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-11-25 12:15:28]

Patchset 1 (id:??) landed as
https://crrev.com/3e6b742e634e657d87c3267c2a8f9158c297f801
Cr-Commit-Position: refs/heads/master@{#361630}

[Sami, 2015-11-26 11:41:32]

A revert of this CL (patchset #1 id:1) has been created in
https://codereview.chromium.org/1474073002/ by skyostil@chromium.org.

The reason for reverting is: Relanding. Looks like the failures may have been
unrelated after all..