Cleanup NetworkActivityIndicatorTabHelper and add unittest.

ios/chrome/browser/web/network_activity_indicator_tab_helper.h

Index: ios/chrome/browser/web/network_activity_indicator_tab_helper.h
diff --git a/ios/chrome/browser/web/network_activity_indicator_tab_helper.h b/ios/chrome/browser/web/network_activity_indicator_tab_helper.h
index a59f63ae57c6a3b9df146058fc883220a50ed95f..c598006f14fdaf13ca405fb0dd312ac2f6cb87a3 100644
--- a/ios/chrome/browser/web/network_activity_indicator_tab_helper.h
+++ b/ios/chrome/browser/web/network_activity_indicator_tab_helper.h
@@ -26,6 +26,9 @@ class NetworkActivityIndicatorTabHelper
   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;