[build-fix] [gcc 4.6.*] Use iterator instead of const_iterator while using std::map::erase(...)

Older versions of gcc, like 4.6.* (which is the default on ubuntu 12.04 LTS), throw build error for using const_iterator.
This has been documented here [1]

[1] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=57158

Review URL: https://codereview.chromium.org/401263002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@284548 0039d316-1c4b-4281-b951-d872f2087c98

mojo/services/window_manager/window_manager_app.cc

@@ -261,7 +261,7 @@ void WindowManagerApp::RegisterSubtree(view_manager::Id id,
 
 void WindowManagerApp::UnregisterSubtree(view_manager::Id id) {
   view_manager::Node* node = view_manager_->GetNodeById(id);
-  NodeIdToWindowMap::const_iterator it = node_id_to_window_map_.find(id);
+  NodeIdToWindowMap::iterator it = node_id_to_window_map_.find(id);
   DCHECK(it != node_id_to_window_map_.end());
   scoped_ptr<aura::Window> window(it->second);
   node_id_to_window_map_.erase(it);