Revert of Revert of Make Reflector Use Main Thread MessageLoop when in SingleThread...

Revert of Revert of Make Reflector Use Main Thread MessageLoop when in SingleThread (patchset #1 id:1 of https://codereview.chromium.org/900133002/)

Reason for revert:
These aren't the changes you're looking for.

Original issue's description:
> Revert of Make Reflector Use Main Thread MessageLoop when in SingleThread (patchset #1 id:1 of https://codereview.chromium.org/899073003/)
>
> Reason for revert:
> Reason for revert:
> Speculatively reverting to try to fix broken Mac 10.8 retina layout tests in Blink:
> compositing/iframes/iframe-composited-scrolling-hide-and-show.html
> compositing/iframes/overlapped-iframe-iframe.html
> svg/as-border-image/svg-as-border-image-2.html
> and more.
>
> Original issue's description:
> > Make Reflector Use Main Thread MessageLoop when in SingleThread
> >
> > When in using single thread with browser compositor, make reflector use
> > main thread MessageLoop instead.
> >
> > Test with forced SW mirrored display on both Linux-cros build and Pixel.
> > BUG=453732
> >
> > Committed: https://crrev.com/0b88b2372f1ad82c12656a4b9a17098470e576a1
> > Cr-Commit-Position: refs/heads/master@{#314665}
>
> TBR=danakj@chromium.org,weiliangc@chromium.org
> NOPRESUBMIT=true
> NOTREECHECKS=true
> NOTRY=true
> BUG=453732
>
> Committed: https://crrev.com/6cb534f690545ebda6751d153a6e72f50d4003cc
> Cr-Commit-Position: refs/heads/master@{#314732}

TBR=danakj@chromium.org,weiliangc@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=453732

Review URL: https://codereview.chromium.org/896393002

Cr-Commit-Position: refs/heads/master@{#314748}