Change the "Terminate" worker-thread handler to an asynchronous function

This is a tiny bit shorter, which cannot hurt.

Author: Snuffleupagus

src/core/worker.js

@@ -956,7 +956,7 @@ class WorkerMessageHandler {
       return pdfManager.cleanup(/* manuallyTriggered = */ true);
     });
 
-    handler.on("Terminate", function (data) {
+    handler.on("Terminate", async function (data) {
       terminated = true;
 
       const waitOn = [];
@@ -977,12 +977,11 @@ class WorkerMessageHandler {
         task.terminate();
       }
 
-      return Promise.all(waitOn).then(function () {
-        // Notice that even if we destroying handler, resolved response promise
-        // must be sent back.
-        handler.destroy();
-        handler = null;
-      });
+      await Promise.all(waitOn);
+      // Notice that even if we destroying handler, resolved response promise
+      // must be sent back.
+      handler.destroy();
+      handler = null;
     });
 
     handler.on("Ready", function (data) {