Bug 1247687 - Handle cancellation of long running modules; r=jonco

This is a slightly annoying thing that can happen. When we abruptly cancel (such as an infinitely
running script being forcibly terminated) we will be in a state where the EvaluateModule call will
finish _after_ the loader is destroyed. So, instead we track if there has been a forcible
cancelation, and exit early.

Depends on D155690

Differential Revision: https://phabricator.services.mozilla.com/D155568

js/loader/ModuleLoaderBase.cpp

@@ -1212,6 +1212,12 @@ nsresult ModuleLoaderBase::EvaluateModuleInContext(
   // unless the user cancels execution.
   MOZ_ASSERT_IF(ok, !JS_IsExceptionPending(aCx));
 
+  // For long running scripts, the request may be cancelled abruptly. This
+  // may also happen if the loader is collected before we get here.
+  if (request->IsCanceled() || !mLoader) {
+    return NS_ERROR_ABORT;
+  }
+
   if (!ok) {
     LOG(("ScriptLoadRequest (%p):   evaluation failed", aRequest));
     // For a dynamic import, the promise is rejected. Otherwise an error is