Do AllocateSharedMemory on FILE_USER_BLOCKING thread.

Do AllocateSharedMemory on FILE_USER_BLOCKING thread.

This operation requires accessing the filesystem on OS X and linux and could be slow.

BUG=466437
Committed: https://crrev.com/153e1323edbd20abe02626afdd29e0135443a047
Cr-Commit-Position: refs/heads/master@{#321267}

[jbauman, 2015-03-18 23:50:20]

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

[jbauman, 2015-03-18 23:50:20]

Obviously not as good as switching to mach memory objects, but this is simple
enough for now and will also help on Linux (though it's not as much of an issue
there).

[Avi (use Gerrit), 2015-03-19 00:07:06]

LGTM

[erikchen, 2015-03-19 00:12:21]

lgtm

[jbauman, 2015-03-19 00:16:01]

The CQ bit was checked by jbauman@chromium.org

[commit-bot: I haz the power, 2015-03-19 00:17:05]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1018953005/1

[commit-bot: I haz the power, 2015-03-19 01:59:35]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-03-19 02:00:32]

Patchset 1 (id:??) landed as
https://crrev.com/153e1323edbd20abe02626afdd29e0135443a047
Cr-Commit-Position: refs/heads/master@{#321267}