Bug 1457606 - use an unambiguous name for GetLoadedModule's promise; r=Dexter

It might be better to address this in nsMainThreadPtrHolder, but it's
also really helpful to have nsMainThreadPtrHolder provide meaningful
names for the main thread runnables.  So we'll address this in
TelemetryImpl itself by providing a more-unique name.

toolkit/components/telemetry/Telemetry.cpp

@@ -999,7 +999,7 @@ TelemetryImpl::GetLoadedModules(JSContext *cx, Promise** aPromise)
   }
 
   nsMainThreadPtrHandle<Promise> mainThreadPromise(
-    new nsMainThreadPtrHolder<Promise>("Promise", promise));
+    new nsMainThreadPtrHolder<Promise>("TelemetryImpl::GetLoadedModules::Promise", promise));
   nsCOMPtr<nsIRunnable> runnable = new GetLoadedModulesRunnable(mainThreadPromise);
   promise.forget(aPromise);