Make existing unit tests only run on GL contexts

tools/gpu/GrContextFactory.h

Index: tools/gpu/GrContextFactory.h
diff --git a/tools/gpu/GrContextFactory.h b/tools/gpu/GrContextFactory.h
index fe828195808491bcdfe8c82688c7999cc1237995..89ca9cadf0903fb73d9baa2f29da19c72f11b5a0 100644
--- a/tools/gpu/GrContextFactory.h
+++ b/tools/gpu/GrContextFactory.h
@@ -80,6 +80,11 @@ public:
         }
     }
 
+    static GrBackend ContextTypeBackend(ContextType type) {
+        // Currently all the context types use the GL backed
+        return kOpenGL_GrBackend;
+    }
+
     static const char* ContextTypeName(ContextType type) {
         switch (type) {
             case kGL_ContextType: