Return GrGLContext from GrTestTarget

BUG=skia:

Review URL: https://codereview.chromium.org/1207393002
diff --git a/tests/ResourceCacheTest.cpp b/tests/ResourceCacheTest.cpp
index f0e7bfa..6f1d404 100644
--- a/tests/ResourceCacheTest.cpp
+++ b/tests/ResourceCacheTest.cpp
@@ -181,7 +181,7 @@
     GrTestTarget tt;
     context->getTestTarget(&tt);
 
-    const GrGLInterface* gl = tt.glInterface();
+    const GrGLInterface* gl = tt.glContext()->interface();
     if (!gl) {
         return;
     }