Fix and simplify InspectorFrontendHost stubs

 - ES6 `class` members are non-enumerable, so `for-in` iterated over an
   empty list, not stubbing anything.
 - When `window.InspectorFrontendHost` was missing, it was unnecessarily
   set to a new object twice.
 - Stub error was shown each time command was invoked; changed to
   showing it once for reduced noise in logs and simpler implementation.

Change-Id: Ib0083737873a373220b3a5e1c0ea18b10482bc6c
Reviewed-on: https://chromium-review.googlesource.com/1068915Reviewed-by: Dmitry Gozman <dgozman@chromium.org>
Commit-Queue: Ingvar Stepanyan <ingvar@cloudflare.com>
Cr-Commit-Position: refs/heads/master@{#592011}

third_party/blink/renderer/devtools/front_end/host/InspectorFrontendHost.js

@@ -556,8 +556,7 @@ Host.InspectorFrontendAPIImpl = class {
 /**
  * @type {!InspectorFrontendHostAPI}
  */
-let InspectorFrontendHost = window.InspectorFrontendHost || null;
-window.InspectorFrontendHost = InspectorFrontendHost;
+let InspectorFrontendHost = window.InspectorFrontendHost;
 (function() {
 
   function initializeInspectorFrontendHost() {
@@ -568,25 +567,17 @@ window.InspectorFrontendHost = InspectorFrontendHost;
     } else {
       // Otherwise add stubs for missing methods that are declared in the interface.
       proto = Host.InspectorFrontendHostStub.prototype;
-      for (const name in proto) {
-        const value = proto[name];
-        if (typeof value !== 'function' || InspectorFrontendHost[name])
+      for (const name of Object.getOwnPropertyNames(proto)) {
+        const stub = proto[name];
+        if (typeof stub !== 'function' || InspectorFrontendHost[name])
           continue;
 
-        InspectorFrontendHost[name] = stub.bind(null, name);
+        console.error(
+            'Incompatible embedder: method InspectorFrontendHost.' + name + ' is missing. Using stub instead.');
+        InspectorFrontendHost[name] = stub;
       }
     }
 
-    /**
-     * @param {string} name
-     * @return {?}
-     */
-    function stub(name) {
-      console.error('Incompatible embedder: method InspectorFrontendHost.' + name + ' is missing. Using stub instead.');
-      const args = Array.prototype.slice.call(arguments, 1);
-      return proto[name].apply(InspectorFrontendHost, args);
-    }
-
     // Attach the events object.
     InspectorFrontendHost.events = new Common.Object();
   }