Add testRunner.runIdleTasks() to force idle tasks to run.

components/test_runner/web_test_delegate.h

 // Copyright 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef COMPONENTS_TEST_RUNNER_WEB_TEST_DELEGATE_H_
 #define COMPONENTS_TEST_RUNNER_WEB_TEST_DELEGATE_H_
 
 #include <string>
 #include <vector>
 
@@ skipping 260 matching lines @@
   virtual void DispatchBeforeInstallPromptEvent(
       int request_id,
       const std::vector<std::string>& event_platforms,
       const base::Callback<void(bool)>& callback) = 0;
 
   virtual blink::WebPlugin* CreatePluginPlaceholder(
     blink::WebLocalFrame* frame,
     const blink::WebPluginParams& params) = 0;
 
   virtual float GetDeviceScaleFactorForTest() const = 0;
+
+  // Run all pending idle tasks, and then run callback.
+  virtual void RunIdleTasks(const base::Closure& callback) = 0;
 };
 
 }  // namespace test_runner
 
 #endif  // COMPONENTS_TEST_RUNNER_WEB_TEST_DELEGATE_H_