Use thread-safe send when using Mojo.

Use thread-safe send when using Mojo.

All the work was done in r325911 by morrita. It looks like when the change landed it was disabled though, so this change just turns it on.

https://groups.google.com/a/chromium.org/d/msg/mojo-dev/gSidpDqtUFE/TY5qeUlD-mIJ has more timing, but the summary is that this makes MojoChannelPerfTest.ChannelProxyPingPong faster by 15% on Windows and roughly 40% on Linux.

In total, ChannelProxyPingPong is faster using Mojo compared to Chrome IPC by roughly 70% on Windows and on Linux it's about the same or slightly faster.

BUG=472113
Committed: https://crrev.com/527a6e1b6bdf00a4d8cb020914067612b6ecde99
Cr-Commit-Position: refs/heads/master@{#340216}

[jam, 2015-07-23 21:56:14]

jam@chromium.org changed reviewers:
+ rockot@chromium.org

[Ken Rockot(use gerrit already), 2015-07-23 21:57:30]

lgtm

[jam, 2015-07-23 23:13:20]

The CQ bit was checked by jam@chromium.org

[commit-bot: I haz the power, 2015-07-23 23:14:24]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1246953003/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1246953003/1

[commit-bot: I haz the power, 2015-07-23 23:52:51]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-07-23 23:54:00]

Patchset 1 (id:??) landed as
https://crrev.com/527a6e1b6bdf00a4d8cb020914067612b6ecde99
Cr-Commit-Position: refs/heads/master@{#340216}