Bug 1296967 - Fix -Wunused-result warning in widget/cocoa/nsCocoaWindow.mm. r=spohl

widget/cocoa/nsCocoaWindow.mm

@@ -516,7 +516,11 @@ nsCocoaWindow::CreatePopupContentView(const LayoutDeviceIntRect &aRect)
   NS_ADDREF(mPopupContentView);
 
   nsIWidget* thisAsWidget = static_cast<nsIWidget*>(this);
-  mPopupContentView->Create(thisAsWidget, nullptr, aRect, nullptr);
+  nsresult rv = mPopupContentView->Create(thisAsWidget, nullptr, aRect,
+                                          nullptr);
+  if (NS_WARN_IF(NS_FAILED(rv))) {
+    return rv;
+  }
 
   ChildView* newContentView = (ChildView*)mPopupContentView->GetNativeData(NS_NATIVE_WIDGET);
   [mWindow setContentView:newContentView];