Revert of Revert of Allow creating context for cc worker thread. (patchset #1 id...

Revert of Revert of Allow creating context for cc worker thread. (patchset #1 id:1 of https://codereview.chromium.org/915523003/)

Reason for revert:
Let's try relanding this again, this time with clobber on mac builder if required -- the change actually looks fine.

Original issue's description:
> Revert of Allow creating context for cc worker thread. (patchset #5 id:80001 of https://codereview.chromium.org/898453007/)
>
> Reason for revert:
> Broke libcontent.dylib build on mac debug builder: http://build.chromium.org/p/chromium.webkit/builders/Mac%20Builder%20%28dbg%29/builds/74405/steps/compile/logs/stdio
>
> Original issue's description:
> > Allow creating context for cc worker thread.
> >
> > An additional context can be provided by the OutputSurface. This will be used for cc/ to create a context on its worker thread.
> >
> > Committed: https://crrev.com/3702467b0a7c261bdf8700597c76e51dc39a449c
> > Cr-Commit-Position: refs/heads/master@{#315469}
>
> TBR=piman@chromium.org,jbauman@chromium.org
> NOPRESUBMIT=true
> NOTREECHECKS=true
> NOTRY=true
>
> Committed: https://crrev.com/66b36fff67a845406d7a2addd0a5f5cd729cb0df
> Cr-Commit-Position: refs/heads/master@{#315552}

TBR=piman@chromium.org,jbauman@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true

Review URL: https://codereview.chromium.org/908853007

Cr-Commit-Position: refs/heads/master@{#315561}