Cleanup NetworkActivityIndicatorTabHelper and add unittest.

ios/chrome/browser/web/network_activity_indicator_tab_helper.h

 // Copyright 2016 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 IOS_CHROME_BROWSER_WEB_NETWORK_ACTIVITY_INDICATOR_TAB_HELPER_H_
 #define IOS_CHROME_BROWSER_WEB_NETWORK_ACTIVITY_INDICATOR_TAB_HELPER_H_
 
 #import <Foundation/Foundation.h>
 
 #import "base/mac/scoped_nsobject.h"
 #include "base/macros.h"
 #include "ios/web/public/web_state/web_state_observer.h"
 #include "ios/web/public/web_state/web_state_user_data.h"
 
 @class NSString;
 
 // Handles listening to webState network activity to control network activity
 // indicator.
 class NetworkActivityIndicatorTabHelper
     : public web::WebStateObserver,
       public web::WebStateUserData<NetworkActivityIndicatorTabHelper> {
  public:
   static void CreateForWebState(web::WebState* web_state, NSString* tab_id);
 
  private:
   NetworkActivityIndicatorTabHelper(web::WebState* web_state, NSString* tab_id);
   ~NetworkActivityIndicatorTabHelper() override;
 
+  // Clears any network activity state associated with this activity.
+  void Stop();
+
   // web::WebStateObserver overrides:
   void DidStartLoading() override;
   void DidStopLoading() override;
 
   // Key used to uniquely identify this activity.
   base::scoped_nsobject<NSString> network_activity_key_;
 
   DISALLOW_COPY_AND_ASSIGN(NetworkActivityIndicatorTabHelper);
 };
 
 #endif  // IOS_CHROME_BROWSER_WEB_NETWORK_ACTIVITY_INDICATOR_TAB_HELPER_H_