Add SkCharacterization creation helper to GrContextThreadSafeProxy (take 2)

TBR=bsalomon@google.com
Change-Id: Id96d4fdbb6889065f10a4a7e0c22a03ad9aa5fef
Reviewed-on: https://skia-review.googlesource.com/107000
Reviewed-by: Robert Phillips <robertphillips@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
diff --git a/src/gpu/mtl/GrMtlCaps.h b/src/gpu/mtl/GrMtlCaps.h
index 935cb07..c23a902 100644
--- a/src/gpu/mtl/GrMtlCaps.h
+++ b/src/gpu/mtl/GrMtlCaps.h
@@ -57,6 +57,11 @@
         return false;
     }
 
+    bool getConfigFromBackendFormat(const GrBackendFormat&, SkColorType,
+                                    GrPixelConfig*) const override {
+        return false;
+    }
+
 private:
     void initFeatureSet(MTLFeatureSet featureSet);