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

ios/chrome/app/application_delegate/metrics_mediator.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/app/application_delegate/metrics_mediator.h"
 
 #include "base/mac/bind_objc_block.h"
 #include "base/metrics/histogram_macros.h"
 #include "base/metrics/user_metrics_action.h"
 #include "base/strings/sys_string_conversions.h"
@@ skipping 126 matching lines @@
   // recording the first user action since fresh start wouldn't be triggered.
   NSDate* lastAppClose = [[NSUserDefaults standardUserDefaults]
       objectForKey:kAppEnteredBackgroundDateKey];
   if (lastAppClose) {
     NSTimeInterval interval = -[lastAppClose timeIntervalSinceNow];
     [startupInformation
         activateFirstUserActionRecorderWithBackgroundTime:interval];
     GURL ntpUrl = GURL(kChromeUINewTabURL);
 
     Tab* currentTab = [[browserViewInformation currentTabModel] currentTab];
-    if (currentTab && [currentTab url] == ntpUrl) {
+    if (currentTab.lastCommittedURL == ntpUrl) {

[kkhorimoto, 2017/04/14 20:28:51]

These metrics are gathered at startup, so I think it's safe to assume that no
load is occurring and that we can use the last committed URL here.

       startupInformation.firstUserActionRecorder->RecordStartOnNTP();
       [startupInformation resetFirstUserActionRecorder];
     } else {
       [startupInformation
           expireFirstUserActionRecorderAfterDelay:kFirstUserActionTimeout];
     }
     // Remove the value so it's not reused if the app crashes.
     [[NSUserDefaults standardUserDefaults]
         removeObjectForKey:kAppEnteredBackgroundDateKey];
   }
@@ skipping 229 matching lines @@
 }
 
 - (BOOL)isMetricsReportingEnabledWifiOnly {
   return GetApplicationContext()->GetLocalState()->GetBoolean(
              metrics::prefs::kMetricsReportingEnabled) &&
          GetApplicationContext()->GetLocalState()->GetBoolean(
              prefs::kMetricsReportingWifiOnly);
 }
 
 @end