fixed Metal release build

Bug: skia:
Change-Id: Iedb7a719f8993ed516bbad6ab3aaadcd4e14b207
Reviewed-on: https://skia-review.googlesource.com/c/162160
Reviewed-by: Ethan Nicholas <ethannicholas@google.com>
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
diff --git a/src/gpu/mtl/GrMtlGpuCommandBuffer.h b/src/gpu/mtl/GrMtlGpuCommandBuffer.h
index db63860..9e1ce45 100644
--- a/src/gpu/mtl/GrMtlGpuCommandBuffer.h
+++ b/src/gpu/mtl/GrMtlGpuCommandBuffer.h
@@ -121,7 +121,9 @@
 
     GrMtlGpu*                                     fGpu;
     // GrRenderTargetProxy bounds
+#ifdef SK_DEBUG
     SkRect                                        fBounds;
+#endif
     GrGpuRTCommandBuffer::LoadAndStoreInfo        fColorLoadAndStoreInfo;
     GrGpuRTCommandBuffer::StencilLoadAndStoreInfo fStencilLoadAndStoreInfo;