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

ios/chrome/app/main_controller.mm

Index: ios/chrome/app/main_controller.mm
diff --git a/ios/chrome/app/main_controller.mm b/ios/chrome/app/main_controller.mm
index 25c25d81a5e68f0154c9c3d02a013151cf9ed176..23f767791b223c20da91c29668cd485cd2bdc05e 100644
--- a/ios/chrome/app/main_controller.mm
+++ b/ios/chrome/app/main_controller.mm
@@ -2229,13 +2229,9 @@ - (Tab*)openOrReuseTabInMode:(ApplicationMode)targetMode
                   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If the visible URL is not the NTP URL, that means that the NTP has been replaced
by the webview.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];