Don't delete the current NavigationEntry when leaving an interstitial page.

BUG=107182
TEST=See bug

Review URL: http://codereview.chromium.org/8976014

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@115189 0039d316-1c4b-4281-b951-d872f2087c98
 void NavigationController::RemoveEntryAtIndexInternal(int index) {
   DCHECK(index < entry_count());
  DCHECK(index != last_committed_entry_index_);
 
   DiscardNonCommittedEntries();
 
   entries_.erase(entries_.begin() + index);
  if (last_committed_entry_index_ > index)
     last_committed_entry_index_--;
 }

 void NavigationController::RemoveEntryAtIndexInternal(int index) {
   DCHECK(index < entry_count());
 
   DiscardNonCommittedEntries();
 
   entries_.erase(entries_.begin() + index);
  if (last_committed_entry_index_ == index)
    last_committed_entry_index_--;
  else if (last_committed_entry_index_ > index)
     last_committed_entry_index_--;
 }
