Add scoped autoreleasepool to tests and examples

This ensures ObjC objects do not leak. On non-Apple
platforms, the scope does nothing.

Fixed: dawn:546
Change-Id: Id43702e6536bf9cb37825a5449511e10cf0734f5
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/55841
Commit-Queue: Austin Eng <enga@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>

src/dawn_native/metal/CommandRecordingContext.mm

@@ -61,7 +61,7 @@ namespace dawn_native { namespace metal {
 
             // The encoder is created autoreleased. Retain it to avoid the autoreleasepool from
             // draining from under us.
-            mBlit = [*mCommands blitCommandEncoder];
+            mBlit.Acquire([[*mCommands blitCommandEncoder] retain]);
         }
         return mBlit.Get();
     }
@@ -82,9 +82,9 @@ namespace dawn_native { namespace metal {
         ASSERT(!mInEncoder);
 
         mInEncoder = true;
-        // The encoder is created autoreleased. Retain it to avoid the autoreleasepool from draining
-        // from under us.
-        mCompute = [*mCommands computeCommandEncoder];
+        // The encoder is created autoreleased. Retain it to avoid the autoreleasepool from
+        // draining from under us.
+        mCompute.Acquire([[*mCommands computeCommandEncoder] retain]);
         return mCompute.Get();
     }
 
@@ -104,9 +104,9 @@ namespace dawn_native { namespace metal {
         ASSERT(!mInEncoder);
 
         mInEncoder = true;
-        // The encoder is created autoreleased. Retain it to avoid the autoreleasepool from draining
-        // from under us.
-        mRender = [*mCommands renderCommandEncoderWithDescriptor:descriptor];
+        // The encoder is created autoreleased. Retain it to avoid the autoreleasepool from
+        // draining from under us.
+        mRender.Acquire([[*mCommands renderCommandEncoderWithDescriptor:descriptor] retain]);
         return mRender.Get();
     }
 

src/dawn_native/metal/DeviceMTL.mm

@@ -308,7 +308,8 @@ namespace dawn_native { namespace metal {
             // The MTLCommandBuffer will be autoreleased by default.
             // The autorelease pool may drain before the command buffer is submitted. Retain so it
             // stays alive.
-            mCommandContext = CommandRecordingContext([*mCommandQueue commandBuffer]);
+            mCommandContext =
+                CommandRecordingContext(AcquireNSPRef([[*mCommandQueue commandBuffer] retain]));
         }
         return &mCommandContext;
     }