Handle correctly start and commit of non-latest navigations.

ios/web/navigation/crw_session_controller.mm

 // Copyright 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #import "ios/web/navigation/crw_session_controller.h"
 
 #include <stddef.h>
 
 #include <algorithm>
 #include <utility>
@@ skipping 523 matching lines @@
   std::swap(mergedItems, _items);
 
   // Update state to reflect inserted NavigationItems.
   _previousItemIndex = -1;
   _lastCommittedItemIndex = self.items.size() - 1;
 
   DCHECK_LT(static_cast<NSUInteger>(_lastCommittedItemIndex),
             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;
 }
 
 - (void)removeItemAtIndex:(NSInteger)index {
   DCHECK(index < static_cast<NSInteger>(self.items.size()));
   DCHECK(index != _lastCommittedItemIndex);
   DCHECK(index >= 0);
 
   [self discardNonCommittedItems];
@@ skipping 83 matching lines @@
   return item;
 }
 
 - (BOOL)isRedirectTransitionForItemAtIndex:(size_t)index {
   DCHECK_LT(index, self.items.size());
   ui::PageTransition transition = self.items[index]->GetTransitionType();
   return (transition & ui::PAGE_TRANSITION_IS_REDIRECT_MASK) ? YES : NO;
 }
 
 @end