Bug 1404035 Address an unused result warning in gfx/thebes/DeviceManagerDx.cpp r=dvander

There's no way to handle this failure, so pass it to Unused.

MozReview-Commit-ID: 37PS9TsPqx7

--HG--
extra : rebase_source : f385f4c50d5407b1929abed7dae123f9570d2205

gfx/thebes/DeviceManagerDx.cpp

@@ -689,7 +689,7 @@ DisableAdvancedLayers(FeatureStatus aStatus, const nsCString aMessage, const nsC
 
   FeatureFailure info(aStatus, aMessage, aFailureId);
   if (GPUParent* gpu = GPUParent::GetSingleton()) {
-    gpu->SendUpdateFeature(Feature::ADVANCED_LAYERS, info);
+    Unused << gpu->SendUpdateFeature(Feature::ADVANCED_LAYERS, info);
   }
 
   if (aFailureId.Length()) {