Revert of Flip flag to turn CrOS Browser Compositor Single Threaded (patchset #5...

Revert of Flip flag to turn CrOS Browser Compositor Single Threaded (patchset #5 id:80001 of https://codereview.chromium.org/781163003/)

Reason for revert:
Possible cause for veyron: Chrome crash in SW Mirror mode (at login screen)

https://code.google.com/p/chromium/issues/detail?id=453732

Original issue's description:
> Flip flag to turn CrOS Browser Compositor Single Threaded
>
> Simply stop using threaded browser compositor for CrOS. Clean up would
> be in follow up patch.
>
> R=danakj@chromium.org, piman@chromium.org
> BUG=329552
>
> Committed: https://crrev.com/e111a7d64d9ba12833900ab743beaf4ee6d267ed
> Cr-Commit-Position: refs/heads/master@{#313842}

TBR=danakj@chromium.org,piman@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=329552

Review URL: https://codereview.chromium.org/893623002

Cr-Commit-Position: refs/heads/master@{#313950}