Revert of Revert of Return GrGLContext from GrTestTarget (patchset #1 id:1 of https...

Revert of Revert of Return GrGLContext from GrTestTarget (patchset #1 id:1 of https://codereview.chromium.org/1211123004/)

Reason for revert:
un-referting, did not help DEPS roll

Original issue's description:
> Revert of Return GrGLContext from GrTestTarget (patchset #3 id:40001 of https://codereview.chromium.org/1207393002/)
>
> Reason for revert:
> speculative revert to unblock DEPS roll (layout-tests)
>
> Original issue's description:
> > Return GrGLContext from GrTestTarget
> >
> > BUG=skia:
> >
> > Committed: https://skia.googlesource.com/skia/+/b6d93ea428ca5d13641c35faca7d0380980a3a17
>
> TBR=bsalomon@google.com,joshualitt@google.com,joshualitt@chromium.org
> NOPRESUBMIT=true
> NOTREECHECKS=true
> NOTRY=true
> BUG=skia:
>
> Committed: https://skia.googlesource.com/skia/+/1852ec2b46a2a0efbb3550ff4d8b640f56a810e9

TBR=bsalomon@google.com,joshualitt@google.com,joshualitt@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=skia:

Review URL: https://codereview.chromium.org/1215593002