Return GrGLContext from GrTestTarget

gm/imagefromyuvtextures.cpp

Index: gm/imagefromyuvtextures.cpp
diff --git a/gm/imagefromyuvtextures.cpp b/gm/imagefromyuvtextures.cpp
index b618abe2a4a9d91cd7a0e1575bba48588b7bfe9b..b39a4dceab250a3f220a933ecc64976b3aae9d09 100644
--- a/gm/imagefromyuvtextures.cpp
+++ b/gm/imagefromyuvtextures.cpp
@@ -105,7 +105,7 @@ protected:
         }
 
         // We currently hav only implemented the texture uploads for GL.
-        const GrGLInterface* gl = tt.glInterface();
+        const GrGLInterface* gl = tt.glContext()->interface();
         if (!gl) {
             return;
         }
@@ -139,7 +139,7 @@ protected:
             return;
         }
 
-        const GrGLInterface* gl = tt.glInterface();
+        const GrGLInterface* gl = tt.glContext()->interface();
         if (!gl) {
             return;
         }