Bug 1540314: Devtools promise actor: Use ActorPool 'destroy' method. r=gl

The prior code calls '.cleanup()', but ActorPools have no 'cleanup' method. I
think it means to call 'destroy'.

Differential Revision: https://phabricator.services.mozilla.com/D29755

--HG--
extra : moz-landing-system : lando

devtools/server/actors/promises.js

@@ -191,7 +191,7 @@ var PromisesActor = protocol.ActorClassWithSpec(promisesSpec, {
       return;
     }
 
-    this._navigationLifetimePool.cleanup();
+    this._navigationLifetimePool.destroy();
     this.dbg.removeAllDebuggees();
     this.dbg.addDebuggees();
   }),