Re-enable ChannelMojo.

Re-enable ChannelMojo.

AGAIN! Previously reverted attempts:

https://codereview.chromium.org/1262503011/
https://codereview.chromium.org/1256473002/

SyncMessageFilter is set up to properly use thread-safe
Send channels now, so this should stick.

Maybe.

BUG=500019
TBR=jam@chromium.org

Committed: https://crrev.com/d34d184bd1a88043667b10cba79a5770bd2803fb
Cr-Commit-Position: refs/heads/master@{#342484}

[Ken Rockot(use gerrit already), 2015-08-06 21:43:40]

Patchset #2 (id:20001) has been deleted

[Ken Rockot(use gerrit already), 2015-08-06 21:48:41]

Patchset #1 (id:1) has been deleted

[Ken Rockot(use gerrit already), 2015-08-07 21:21:08]

The CQ bit was checked by rockot@chromium.org

[commit-bot: I haz the power, 2015-08-07 21:21:50]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1273283002/40001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1273283002/40001

[commit-bot: I haz the power, 2015-08-07 23:59:55]

Committed patchset #1 (id:40001)

[commit-bot: I haz the power, 2015-08-08 00:00:54]

Patchset 1 (id:??) landed as
https://crrev.com/d34d184bd1a88043667b10cba79a5770bd2803fb
Cr-Commit-Position: refs/heads/master@{#342484}

[Ken Rockot(use gerrit already), 2015-08-08 14:29:51]

A revert of this CL (patchset #1 id:40001) has been created in
https://codereview.chromium.org/1285463002/ by rockot@chromium.org.

The reason for reverting is: Still seeing flake on ChromeOS, and it's probably
this again..