Remove NavigationManager::GetPreviousItem().

ios/chrome/browser/tabs/tab_model.mm

Index: ios/chrome/browser/tabs/tab_model.mm
diff --git a/ios/chrome/browser/tabs/tab_model.mm b/ios/chrome/browser/tabs/tab_model.mm
index 0a4a9fdf5b0dbfd915085c012970856134f9335a..799264cfe86e4be24f19804aa164ac6bb3617a51 100644
--- a/ios/chrome/browser/tabs/tab_model.mm
+++ b/ios/chrome/browser/tabs/tab_model.mm
@@ -757,15 +757,14 @@ Tab* GetOpenerForTab(id<NSFastEnumeration> tabs, Tab* tab) {
     [self saveSessionImmediately:NO];
 }
 
-- (void)navigationCommittedInTab:(Tab*)tab {
+- (void)navigationCommittedInTab:(Tab*)tab
+                    previousItem:(web::NavigationItem*)previousItem {
   if (self.offTheRecord)
     return;
   if (![tab navigationManager])
     return;
 
   // See if the navigation was within a page; if so ignore it.
-  web::NavigationItem* previousItem =
-      [tab navigationManagerImpl]->GetPreviousItem();
   if (previousItem) {
     GURL previousURL = previousItem->GetURL();
     GURL currentURL = [tab navigationManager]->GetVisibleItem()->GetURL();