Issue #3020 - Clear currentScript after running microtasks

Per HTML spec:

`document.currentScript` Returns null if the Document is not currently
executing a script or SVG script element (e.g., because the running
script is an event handler, or a timeout), or if the currently executing
script or SVG script element represents a module script.

---

It's ambiguous about whether, for micro tasks, it should return null.
(This section is labelled as "non-normative", as well.)
However, mainstream browsers handle this differently and don't return
null after running microtasks, so we need to clear this and not assume
null persistently, and run the auto-updater for it, regardless if
it's a module or not.

dom/script/ScriptLoader.cpp

@@ -2315,8 +2315,14 @@ ScriptLoader::EvaluateScript(ScriptLoadRequest* aRequest)
     return NS_ERROR_FAILURE;
   }
 
+  // Update our current script.
+  // This must be destroyed after destroying nsAutoMicroTask.
+  nsIScriptElement* currentScript =
+      aRequest->IsModuleRequest() ? nullptr : aRequest->Element();
+  AutoCurrentScriptUpdater scriptUpdater(this, currentScript);
+
   // New script entry point required, due to the "Create a script" sub-step of
-  // http://www.whatwg.org/specs/web-apps/current-work/#execute-the-script-block
+  // http://html.spec.whatwg.org/multipage/#execute-the-script-block
   nsAutoMicroTask mt;
   AutoEntryScript aes(globalObject, "<script> element", true);
   JSContext* cx = aes.cx();
@@ -2374,9 +2380,6 @@ ScriptLoader::EvaluateScript(ScriptLoadRequest* aRequest)
         FinishDynamicImport(cx, request, rv);
       }
     } else {
-      // Update our current script.
-      AutoCurrentScriptUpdater scriptUpdater(this, aRequest->Element());
-
       JS::CompileOptions options(cx);
       rv = FillCompileOptionsForRequest(aes, aRequest, global, &options);