Re-enable ChannelMojo

Re-enable ChannelMojo

The performance regression seen previously was probably
misattributed to ChannelMojo. Also, given that a great deal of
code will now take advantage of the channel's thread-safe Send,
this is expected to result in an net performance improvement.

BUG=500019
TBR=jam@chromium.org

Committed: https://crrev.com/ab1a53c964a77dabb6d08b7199111b51a330885e
Cr-Commit-Position: refs/heads/master@{#342055}

[Ken Rockot(use gerrit already), 2015-08-05 04:47:46]

Patchset #1 (id:1) has been deleted

[Ken Rockot(use gerrit already), 2015-08-06 03:38:11]

The CQ bit was checked by rockot@chromium.org

[commit-bot: I haz the power, 2015-08-06 03:38:22]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1262503011/20001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1262503011/20001

[commit-bot: I haz the power, 2015-08-06 03:43:59]

Committed patchset #1 (id:20001)

[commit-bot: I haz the power, 2015-08-06 03:44:53]

Patchset 1 (id:??) landed as
https://crrev.com/ab1a53c964a77dabb6d08b7199111b51a330885e
Cr-Commit-Position: refs/heads/master@{#342055}

[hiroshige, 2015-08-06 09:27:39]

A revert of this CL (patchset #1 id:20001) has been created in
https://codereview.chromium.org/1265383006/ by hiroshige@chromium.org.

The reason for reverting is: Suspecting this CL causes many Layout test failures
(https://crbug.com/517391)..