Metal stencil support

Bug: skia:
Change-Id: I697a39bc756bb8f18ea2c6a1f8151dbc103bf9fe
Reviewed-on: https://skia-review.googlesource.com/c/157567
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
diff --git a/src/gpu/mtl/GrMtlGpuCommandBuffer.h b/src/gpu/mtl/GrMtlGpuCommandBuffer.h
index d3f964f..83efb07 100644
--- a/src/gpu/mtl/GrMtlGpuCommandBuffer.h
+++ b/src/gpu/mtl/GrMtlGpuCommandBuffer.h
@@ -85,7 +85,7 @@
 
     void onClear(const GrFixedClip& clip, GrColor color) override {}
 
-    void onClearStencilClip(const GrFixedClip& clip, bool insideStencilMask) override {}
+    void onClearStencilClip(const GrFixedClip& clip, bool insideStencilMask) override;
 
     MTLRenderPassDescriptor* createRenderPassDesc() const;