grit: Add fallback_to_default_layout attribute to <output>

grit/node/structure.py

 #!/usr/bin/env python
 # 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.
 
 '''The <structure> element.
 '''
 
 import os
 import platform
@@ skipping 273 matching lines @@
       create_file: True
     '''
     assert self.HasFileForLanguage()
     # If the source language is requested, and no extra changes are requested,
     # use the existing file.
     if ((not lang or lang == self.GetRoot().GetSourceLanguage()) and
         self.attrs['expand_variables'] != 'true' and
         (not self.attrs['run_command'] or
          not self.RunCommandOnCurrentPlatform())):
       if return_if_not_generated:
-        return self.ToRealPath(self.GetInputPath())
+        input_path = self.GetInputPath()
+        if input_path is None:
+          return None
+        return self.ToRealPath(input_path)
       else:
         return None
 
     if self.attrs['output_filename'] != '':
       filename = self.attrs['output_filename']
     else:
       filename = os.path.basename(self.attrs['file'])
     assert len(filename)
     filename = '%s_%s' % (lang, filename)
     filename = os.path.join(output_dir, filename)
@@ skipping 53 matching lines @@
   def SubstituteMessages(self, substituter):
     '''Propagates substitution to gatherer.
 
     Args:
       substituter: a grit.util.Substituter object.
     '''
     assert hasattr(self, 'gatherer')
     if self.ExpandVariables():
       self.gatherer.SubstituteMessages(substituter)