[graphite] Fix commandqueue memory leak

Bug: skia:12466
Change-Id: Icbd4fd6098c8a50164613e630db476bf8ea41517
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/459177
Auto-Submit: Jim Van Verth <jvanverth@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>
diff --git a/tools/graphite/mtl/MtlTestContext.mm b/tools/graphite/mtl/MtlTestContext.mm
index 7085f28..60de85a 100644
--- a/tools/graphite/mtl/MtlTestContext.mm
+++ b/tools/graphite/mtl/MtlTestContext.mm
@@ -42,7 +42,7 @@
 
     skgpu::mtl::BackendContext backendContext = {};
     backendContext.fDevice.retain(device.get());
-    backendContext.fQueue.retain([*device newCommandQueue]);
+    backendContext.fQueue.reset([*device newCommandQueue]);
 
     return std::unique_ptr<GraphiteTestContext>(new TestContext(backendContext));
 }