Cleanup NetworkActivityIndicatorTabHelper and add unittest.

ios/chrome/browser/web/network_activity_indicator_tab_helper.mm

 // 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.
 
 #import "ios/chrome/browser/web/network_activity_indicator_tab_helper.h"
 
 #import "ios/chrome/browser/ui/network_activity_indicator_manager.h"
 #import "ios/web/public/web_state/web_state.h"
 
 DEFINE_WEB_STATE_USER_DATA_KEY(NetworkActivityIndicatorTabHelper);
 
 // static
 void NetworkActivityIndicatorTabHelper::CreateForWebState(
     web::WebState* web_state,
     NSString* tab_id) {
   DCHECK(web_state);
   if (!FromWebState(web_state)) {
     web_state->SetUserData(UserDataKey(), new NetworkActivityIndicatorTabHelper(
                                               web_state, tab_id));
   }
 }
 
 NetworkActivityIndicatorTabHelper::NetworkActivityIndicatorTabHelper(
     web::WebState* web_state,
     NSString* tab_id)
     : web::WebStateObserver(web_state), network_activity_key_([tab_id copy]) {}
 
 NetworkActivityIndicatorTabHelper::~NetworkActivityIndicatorTabHelper() {
-  NetworkActivityIndicatorManager* shared_manager =
-      [NetworkActivityIndicatorManager sharedInstance];
-  // Verifies that there is a network task associated with this instance
-  // before stopping a task, so that this method is idempotent.
-  if ([shared_manager numNetworkTasksForGroup:network_activity_key_])
-    [shared_manager stopNetworkTaskForGroup:network_activity_key_];
+  Stop();
 }
 
 void NetworkActivityIndicatorTabHelper::DidStartLoading() {
   NetworkActivityIndicatorManager* shared_manager =
       [NetworkActivityIndicatorManager sharedInstance];
   // Verifies that there are not any network tasks associated with this instance
   // before starting another task, so that this method is idempotent.
   if (![shared_manager numNetworkTasksForGroup:network_activity_key_])
     [shared_manager startNetworkTaskForGroup:network_activity_key_];
 }
 
 void NetworkActivityIndicatorTabHelper::DidStopLoading() {
+  Stop();
+}
+
+void NetworkActivityIndicatorTabHelper::Stop() {
   NetworkActivityIndicatorManager* shared_manager =
       [NetworkActivityIndicatorManager sharedInstance];
   // Verifies that there is a network task associated with this instance
   // before stopping a task, so that this method is idempotent.
   if ([shared_manager numNetworkTasksForGroup:network_activity_key_])
     [shared_manager stopNetworkTaskForGroup:network_activity_key_];
 }