Bug 1554072 - Only crash in debug builds when using createElement with XUL. r=bzbarsky

Temporary solution until we get sanitized chrome JS stacks for the crash
reporter.

Differential Revision: https://phabricator.services.mozilla.com/D32933

--HG--
extra : moz-landing-system : lando

dom/base/Document.cpp

@@ -5545,8 +5545,7 @@ already_AddRefed<Element> Document::CreateElement(
 #if DEBUG
     xpc_DumpJSStack(true, true, false);
 #endif
-    MOZ_DIAGNOSTIC_ASSERT(false,
-                          "CreateElement() not allowed in XUL document.");
+    MOZ_ASSERT_UNREACHABLE("CreateElement() not allowed in XUL document.");
     rv.Throw(NS_ERROR_FAILURE);
     return nullptr;
   }