Remove usage of Tab's |url| property from MainController.

ios/chrome/app/main_controller.mm

 // Copyright 2012 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/app/main_controller.h"
 
 #include <memory>
 #include <string>
 
 #import <CoreSpotlight/CoreSpotlight.h>
@@ skipping 2211 matching lines @@
   }
 }
 
 #pragma mark - Tab opening utility methods.
 
 - (Tab*)openOrReuseTabInMode:(ApplicationMode)targetMode
                      withURL:(const GURL&)URL
                   transition:(ui::PageTransition)transition {
   BrowserViewController* targetBVC =
       targetMode == ApplicationMode::NORMAL ? self.mainBVC : self.otrBVC;
-  GURL currentURL;
 
   Tab* currentTabInTargetBVC = [[targetBVC tabModel] currentTab];
-  if (currentTabInTargetBVC)
-    currentURL = [currentTabInTargetBVC url];
-
-  if (!(currentTabInTargetBVC && IsURLNtp(currentURL))) {
+  if (!(currentTabInTargetBVC && IsURLNtp(currentTabInTargetBVC.visibleURL))) {

[kkhorimoto, 2017/04/14 20:27:23]

We want to return the current Tab if no navigations have occurred.  If there's a
user-initiated load on an NTP, we would want to create a new Tab, so using
visible URL here.

[Eugene But (OOO till 7-30), 2017/04/14 21:38:36]

But NTP can not be pending, or transient, right? If so, then lastCommittedURL
should work here, I guess....

[kkhorimoto, 2017/05/31 23:02:08]

If the visible URL is not the NTP URL, that means that the NTP has been replaced
by the webview.  The intention of this code is to open the url in the current
tab only if it's displaying the NTP, which will only be true if the visible URL
is the NTP.

     return [targetBVC addSelectedTabWithURL:URL
                                     atIndex:NSNotFound
                                  transition:transition];
   }
 
   Tab* newTab = currentTabInTargetBVC;
   // Don't call loadWithParams for chrome://newtab, it's already loaded.
   if (!(IsURLNtp(URL))) {
     web::NavigationManager::WebLoadParams params(URL);
     [newTab webState]->GetNavigationManager()->LoadURLWithParams(params);
@@ skipping 364 matching lines @@
   };
 
   callbackCounter->IncrementCount();
   [self removeBrowsingDataFromBrowserState:_mainBrowserState
                                       mask:removeAllMask
                                 timePeriod:browsing_data::TimePeriod::ALL_TIME
                          completionHandler:decrementCallbackCounterCount];
 }
 
 @end