[iOS] Restore the user agent only on iOS 13+

This CL restrict the user agent restoration to iOS 13 and further as
iOS 12 has an issue getting the right User Agent.

Bug: 1070593
Change-Id: Ie4570efcb9931ccb8662511b16a1298570f0a297
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2154189Reviewed-by: Justin Cohen <justincohen@chromium.org>
Commit-Queue: Gauthier Ambard <gambard@chromium.org>
Cr-Commit-Position: refs/heads/master@{#760057}

ios/web/navigation/wk_based_navigation_manager_impl.mm

@@ -741,12 +741,16 @@ void WKBasedNavigationManagerImpl::UnsafeRestore(
     forward_items.push_back(std::move(items[index]));
   }
 
+  if (@available(iOS 13, *)) {
+    AddRestoreCompletionCallback(
+        base::BindOnce(&WKBasedNavigationManagerImpl::RestoreItemsState,
+                       base::Unretained(this), RestoreItemListType::kBackList,
+                       std::move(back_items)));
     AddRestoreCompletionCallback(base::BindOnce(
-      &WKBasedNavigationManagerImpl::RestoreItemsState, base::Unretained(this),
-      RestoreItemListType::kBackList, std::move(back_items)));
-  AddRestoreCompletionCallback(base::BindOnce(
-      &WKBasedNavigationManagerImpl::RestoreItemsState, base::Unretained(this),
-      RestoreItemListType::kForwardList, std::move(forward_items)));
+        &WKBasedNavigationManagerImpl::RestoreItemsState,
+        base::Unretained(this), RestoreItemListType::kForwardList,
+        std::move(forward_items)));
+  }
 
   LoadURLWithParams(params);
 
@@ -795,9 +799,11 @@ void WKBasedNavigationManagerImpl::RestoreItemsState(
 void WKBasedNavigationManagerImpl::RestoreVisibleItemState() {
   NavigationItemImpl* last_committed_item =
       GetLastCommittedItemInCurrentOrRestoredSession();
+  if (@available(iOS 13, *)) {
     if (restored_visible_item_ && last_committed_item) {
       last_committed_item->RestoreStateFromItem(restored_visible_item_.get());
     }
+  }
   restored_visible_item_.reset();
 }