Don't release things which will be autoreleased.

src/backend/metal/CommandBufferMTL.mm

@@ -43,8 +43,7 @@ namespace backend { namespace metal {
                 ASSERT(compute == nil);
                 if (blit != nil) {
                     [blit endEncoding];
-                    [blit release];
+                    blit = nil;  // This will be autoreleased.
-                    blit = nil;
                 }
             }
 
@@ -65,16 +64,14 @@ namespace backend { namespace metal {
             void EndCompute() {
                 ASSERT(compute != nil);
                 [compute endEncoding];
-                [compute release];
+                compute = nil;  // This will be autoreleased.
-                compute = nil;
             }
 
             void BeginSubpass(id<MTLCommandBuffer> commandBuffer, uint32_t subpass) {
                 ASSERT(currentRenderPass);
                 if (render != nil) {
                     [render endEncoding];
-                    [render release];
+                    render = nil;  // This will be autoreleased.
-                    render = nil;
                 }
 
                 const auto& info = currentRenderPass->GetSubpassInfo(subpass);
@@ -144,15 +141,13 @@ namespace backend { namespace metal {
                 }
 
                 render = [commandBuffer renderCommandEncoderWithDescriptor:descriptor];
-                [descriptor release];
                 // TODO(cwallez@chromium.org): does any state need to be reset?
             }
 
             void EndSubpass() {
                 ASSERT(render != nil);
                 [render endEncoding];
-                [render release];
+                render = nil;  // This will be autoreleased.
-                render = nil;
             }
         };
     }

src/backend/metal/MetalBackend.mm

@@ -62,8 +62,7 @@ namespace backend { namespace metal {
         }
         Tick();
 
-        [mPendingCommands release];
+        mPendingCommands = nil;  // This will be autoreleased.
-        mPendingCommands = nil;
 
         delete mMapTracker;
         mMapTracker = nullptr;
@@ -171,8 +170,7 @@ namespace backend { namespace metal {
         }];
 
         [mPendingCommands commit];
-        [mPendingCommands release];
+        mPendingCommands = nil;  // This will be autoreleased.
-        mPendingCommands = nil;
         mPendingCommandSerial++;
     }