Fix Revert of Make ui::Compositor use ui::Scheduler (patchset #2 id:20001 of https...

Fix Revert of Make ui::Compositor use ui::Scheduler (patchset #2 id:20001 of https://codereview.chromium.org/638653003/)

This fixes the revert.

Reason for revert:
Speculatively reverting as I suspect it may be the cause of test failures like this:

[16079:16079:1009/052004:FATAL:compositor.cc(303)] Check failed: !IsLocked().

http://build.chromium.org/p/chromium.linux/builders/Linux%20Tests%20%28dbg%29%281%29%2832%29/builds/6451

Original issue's description:
> Make ui::Compositor use ui::Scheduler
>
> Taken from enne's CL 535733002 and rebased. It has been taken out of CL 134623005.
>
> BUG=329552
>
> Committed: https://crrev.com/36b7fc7f8b05ea627873e58a162c1c26784e472d
> Cr-Commit-Position: refs/heads/master@{#298779}

TBR=ben@chromium.org,danakj@chromium.org,sky@chromium.org,weiliangc@chromium.org
NOTREECHECKS=true
NOTRY=true
BUG=329552

Committed: https://crrev.com/1f27b28c347712516bac2454bfece4beeb581568
Cr-Commit-Position: refs/heads/master@{#298919}

Review URL: https://codereview.chromium.org/645553003

Cr-Commit-Position: refs/heads/master@{#298922}