Add Autorelease pools to clear out no longer needed Metal objects.

Bug: skia:8822
Change-Id: I2bb3528100a54a169ec9c5b26ef7d4d1cacc19db
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/198244
Reviewed-by: Greg Daniel <egdaniel@google.com>
Reviewed-by: Ethan Nicholas <ethannicholas@google.com>
Commit-Queue: Jim Van Verth <jvanverth@google.com>
diff --git a/src/gpu/mtl/GrMtlGpuCommandBuffer.h b/src/gpu/mtl/GrMtlGpuCommandBuffer.h
index dc50dd6..f673354 100644
--- a/src/gpu/mtl/GrMtlGpuCommandBuffer.h
+++ b/src/gpu/mtl/GrMtlGpuCommandBuffer.h
@@ -125,8 +125,8 @@
     GrGpuRTCommandBuffer::LoadAndStoreInfo        fColorLoadAndStoreInfo;
     GrGpuRTCommandBuffer::StencilLoadAndStoreInfo fStencilLoadAndStoreInfo;
 
-    __strong id<MTLRenderCommandEncoder> fActiveRenderCmdEncoder;
-    MTLRenderPassDescriptor* fRenderPassDesc;
+    id<MTLRenderCommandEncoder> fActiveRenderCmdEncoder;
+    MTLRenderPassDescriptor*    fRenderPassDesc;
 
     struct CommandBufferInfo {
         SkRect fBounds;