Revert of Re-enable ChannelMojo (patchset #1 id:80001 of https://codereview.chromium...

Revert of Re-enable ChannelMojo (patchset #1 id:80001 of https://codereview.chromium.org/1256473002/)

Reason for revert:
Maybe breaking layout tests...somehow

Original issue's description:
> Re-enable ChannelMojo
>
> I'm pretty sure the previous perf regression was a false
> signal. Haven't reproed with other benchmarks and in
> retrospect turning ChannelMojo off never fixed the
> regression in question anyway.
>
> BUG=500019
>
> Committed: https://crrev.com/ccfe630130d0b4768338a88361c98cc8fc35f758
> Cr-Commit-Position: refs/heads/master@{#340975}

TBR=jam@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=500019

Review URL: https://codereview.chromium.org/1256033004

Cr-Commit-Position: refs/heads/master@{#341001}