Bug 1678774 - Cancel off-thread compilations when destroying a ScriptLoader r=dpalmeiro

There are parse tasks present when the JS engine is being shut down that have
finished but haven't had their results collected by the embedding. This
shouldn't happen, and I think it's happening here because we're leaking a
JSRuntime (we cancel these when we shut down a runtime).

There's a comment that says this isn't necessary but cancelling outstanding
compile requests in ScriptLoader::Destroy fixes this problem on try. I don't
understand well enough to know what's going wrong with the current approach but
this fixes both the crash and the leak.

Differential Revision: https://phabricator.services.mozilla.com/D119315

dom/script/ScriptLoadRequest.cpp

@@ -150,8 +150,8 @@ void ScriptLoadRequest::SetReady() {
 }
 
 void ScriptLoadRequest::Cancel() {
-  MaybeCancelOffThreadScript();
   mIsCanceled = true;
+  MaybeCancelOffThreadScript();
 }
 
 void ScriptLoadRequest::MaybeCancelOffThreadScript() {