Added NavigationManager::GetIndexOfItem().

ios/web/public/test/fakes/test_navigation_manager.mm

Index: ios/web/public/test/fakes/test_navigation_manager.mm
diff --git a/ios/web/public/test/fakes/test_navigation_manager.mm b/ios/web/public/test/fakes/test_navigation_manager.mm
index 972efdcac3ce063fbf2893d2ae9e8bc60fb1007a..c1c0125b1ffd7b5bec3a2d1af9ff993006c360b1 100644
--- a/ios/web/public/test/fakes/test_navigation_manager.mm
+++ b/ios/web/public/test/fakes/test_navigation_manager.mm
@@ -75,6 +75,15 @@
   return items_[index].get();
 }
 
+int TestNavigationManager::GetIndexOfItem(
+    const web::NavigationItem* item) const {
+  for (size_t index = 0; index < items_.size(); ++index) {
+    if (items_[index].get() == item)
+      return index;
+  }
+  return -1;
+}
+
 void TestNavigationManager::SetLastCommittedItemIndex(const int index) {
   DCHECK(index == -1 || index >= 0 && index < GetItemCount());
   items_index_ = index;