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:

Review URL: https://codereview.chromium.org/1211123004
8 files changed