Bug 1298149 - explicitly ignore the result of SendUpdateWindow in ContentParent::SendAsyncUpdate; r=baku

This change doesn't modify the behavior of the code, but it does silence
a clang-cl warning.

dom/ipc/ContentParent.cpp

@@ -850,7 +850,7 @@ ContentParent::SendAsyncUpdate(nsIWidget* aWidget)
   ContentParent* cp = reinterpret_cast<ContentParent*>(
     ::GetPropW(hwnd, kPluginWidgetContentParentProperty));
   if (cp && !cp->IsDestroyed()) {
-    cp->SendUpdateWindow((uintptr_t)hwnd);
+    Unused << cp->SendUpdateWindow((uintptr_t)hwnd);
   }
 }
 #endif // defined(XP_WIN)