NavigationController: Fix several broken class invariants.

content/public/browser/navigation_controller.h

 // Copyright (c) 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.
 
 #ifndef CONTENT_PUBLIC_BROWSER_NAVIGATION_CONTROLLER_H_
 #define CONTENT_PUBLIC_BROWSER_NAVIGATION_CONTROLLER_H_
 
 #include <stdint.h>
 
 #include <map>
@@ skipping 250 matching lines @@
 
   // Returns the index from which we would go back/forward or reload.  This is
   // the last_committed_entry_index_ if pending_entry_index_ is -1.  Otherwise,
   // it is the pending_entry_index_.
   virtual int GetCurrentEntryIndex() const = 0;
 
   // Returns the last committed entry, which may be null if there are no
   // committed entries.
   virtual NavigationEntry* GetLastCommittedEntry() const = 0;
 
-  // Returns the index of the last committed entry.
+  // Returns the index of the last committed entry.  It will be -1 iff there are
+  // no entries.
   virtual int GetLastCommittedEntryIndex() const = 0;
 
   // Returns true if the source for the current entry can be viewed.
   virtual bool CanViewSource() const = 0;
 
   // Navigation list -----------------------------------------------------------
 
   // Returns the number of entries in the NavigationController, excluding
   // the pending entry if there is one, but including the transient entry if
   // any.
@@ skipping 173 matching lines @@
 
  private:
   // This interface should only be implemented inside content.
   friend class NavigationControllerImpl;
   NavigationController() {}
 };
 
 }  // namespace content
 
 #endif  // CONTENT_PUBLIC_BROWSER_NAVIGATION_CONTROLLER_H_