Added NavigationManager::GetIndexOfItem().

ios/web/navigation/navigation_manager_impl.mm

Index: ios/web/navigation/navigation_manager_impl.mm
diff --git a/ios/web/navigation/navigation_manager_impl.mm b/ios/web/navigation/navigation_manager_impl.mm
index f7c69ed03d22b7b3853b8c8996b205237db95850..a4e073289f53cdb6e63fec01a40eb0e98263ac89 100644
--- a/ios/web/navigation/navigation_manager_impl.mm
+++ b/ios/web/navigation/navigation_manager_impl.mm
@@ -267,6 +267,11 @@ bool AreURLsInPageNavigation(const GURL& existing_url, const GURL& new_url) {
   return [session_controller_ itemAtIndex:index];
 }
 
+int NavigationManagerImpl::GetIndexOfItem(
+    const web::NavigationItem* item) const {
+  return [session_controller_ indexOfItem:item];
+}
+
 int NavigationManagerImpl::GetPendingItemIndex() const {
   if (GetPendingItem()) {
     if ([session_controller_ pendingItemIndex] != -1) {