Bug 1569676 - Make DOM and application panel's destroy synchronous. r=jdescottes

Differential Revision: https://phabricator.services.mozilla.com/D39709

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

devtools/client/application/panel.js

@@ -38,8 +38,6 @@ class ApplicationPanel {
     this.panelWin = null;
     this.toolbox = null;
     this.emit("destroyed");
-
-    return this;
   }
 }
 

devtools/client/dom/panel.js

@@ -89,19 +89,15 @@ DomPanel.prototype = {
   },
 
   destroy() {
-    if (this._destroying) {
-      return this._destroying;
+    if (this._destroyed) {
+      return;
     }
+    this._destroyed = true;
 
-    this._destroying = new Promise(resolve => {
-      this.target.off("navigate", this.onTabNavigated);
-      this._toolbox.off("select", this.onPanelVisibilityChange);
+    this.target.off("navigate", this.onTabNavigated);
+    this._toolbox.off("select", this.onPanelVisibilityChange);
 
-      this.emit("destroyed");
-      resolve();
-    });
-
-    return this._destroying;
+    this.emit("destroyed");
   },
 
   // Events