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
diff --git a/tests/ResourceCacheTest.cpp b/tests/ResourceCacheTest.cpp
index 6f1d404..f0e7bfa 100644
--- a/tests/ResourceCacheTest.cpp
+++ b/tests/ResourceCacheTest.cpp
@@ -181,7 +181,7 @@
     GrTestTarget tt;
     context->getTestTarget(&tt);
 
-    const GrGLInterface* gl = tt.glContext()->interface();
+    const GrGLInterface* gl = tt.glInterface();
     if (!gl) {
         return;
     }