Handle correctly start and commit of non-latest navigations.

ios/web/navigation/crw_session_controller.mm

Index: ios/web/navigation/crw_session_controller.mm
diff --git a/ios/web/navigation/crw_session_controller.mm b/ios/web/navigation/crw_session_controller.mm
index cf5175e1673d9b538cc846748aede176d338b9c8..58d095a83c8377d39736ad1162c309865a44a407 100644
--- a/ios/web/navigation/crw_session_controller.mm
+++ b/ios/web/navigation/crw_session_controller.mm
@@ -541,21 +541,26 @@ initiationType:(web::NavigationInitiationType)initiationType;
             self.items.size());
 }
 
-- (void)goToItemAtIndex:(NSInteger)index {
+- (void)goToItemAtIndex:(NSInteger)index
+    discardNonCommittedItems:(BOOL)discard {
   if (index < 0 || static_cast<NSUInteger>(index) >= self.items.size())
     return;
 
-  if (index < _lastCommittedItemIndex) {
-    // Going back.
-    [self discardNonCommittedItems];
-  } else if (_lastCommittedItemIndex < index) {
-    // Going forward.
-    [self discardTransientItem];
-  } else {
-    // |delta| is 0, no need to change the last committed item index.
+  if (index == _lastCommittedItemIndex) {
+    // |delta| is 0, no need to change current navigation index.
     return;
   }
 
+  if (discard) {
+    if (index < _lastCommittedItemIndex) {
+      // Going back.
+      [self discardNonCommittedItems];
+    } else if (_lastCommittedItemIndex < index) {
+      // Going forward.
+      [self discardTransientItem];
+    }
+  }
+
   _previousItemIndex = _lastCommittedItemIndex;
   _lastCommittedItemIndex = index;
 }