implemented getting format from texture as virtual in gpu caps

Bug: skia:
Change-Id: If6bbbd212ff472ea322d2bbed61995fe7ba85df7
Reviewed-on: https://skia-review.googlesource.com/138240
Commit-Queue: Timothy Liang <timliang@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
Reviewed-by: Greg Daniel <egdaniel@google.com>
diff --git a/src/gpu/mtl/GrMtlCaps.h b/src/gpu/mtl/GrMtlCaps.h
index 0918f36..1ff16ad 100644
--- a/src/gpu/mtl/GrMtlCaps.h
+++ b/src/gpu/mtl/GrMtlCaps.h
@@ -75,6 +75,11 @@
 
     void initGrCaps(const id<MTLDevice> device);
     void initShaderCaps();
+
+#ifdef GR_TEST_UTILS
+    GrBackendFormat onCreateFormatFromBackendTexture(const GrBackendTexture&) const override;
+#endif
+
     void initConfigTable();
 
     struct ConfigInfo {