WebdriverAgent & Agent: async destroy() and stop returns boolean

lib/Agent.js

@@ -31,7 +31,7 @@ class Agent {
 
   // defaults that do nothing
   async initialize() { return this; }
-  destroy() {}
+  async destroy() {}
   stop() {}
 }
 

lib/ConsoleAgent.js

@@ -265,7 +265,7 @@ class ConsoleAgent extends Agent {
   }
 
   // console agents need to kill the process before exiting.
-  destroy() {
+  async destroy() {
     return this.stop();
   }
 

lib/WebdriverAgent.js

@@ -49,19 +49,21 @@ class WebdriverAgent extends BrowserAgent {
       .then(() => Promise.all([quitP, getP]));
     return this._restartP.then(() => {
       this._restartP = null;
-      return undefined;
+      return true;
     });
   }
 
-  destroy() {
+  async destroy() {
     let baseP = Promise.resolve();
     if (this._restartP) {
       // selenium doesn't like to exit properly if you try to kill while it's doing something
       // so, let's wait.
       baseP = this._restartP;
     }
-
-    return baseP.then(() => this._driver.quit()).then(() => super.destroy());
+    await baseP;
+    await this._driver.quit();
+    await this._driver.quit();
+    return super.destroy();
   }
 }