Fix a couple small problems with tools/unused-grit-header.py.

tools/unused-grit-header.py

 #!/usr/bin/env python
 # Copyright 2014 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.
 
 """A tool to scan source files for unneeded grit includes."""
 
 import os
 import sys
 import xml.etree.ElementTree
@@ skipping 60 matching lines @@
     grd_file: The file that contains the XML tree.
 
   Returns:
     A list of resource names.
   """
   root = tree.getroot()
   assert root.tag == 'grit'
   release_node = FindNodeWithTag(root, 'release')
   assert release_node != None
 
-  messages_node = FindNodeWithTag(root, 'messages')
+  messages_node = FindNodeWithTag(release_node, 'messages')
   messages = set()
   if messages_node != None:
     messages = set(GetResourcesForNode(messages_node, grd_file, 'message'))
 
-  includes_node = FindNodeWithTag(root, 'includes')
+  includes_node = FindNodeWithTag(release_node, 'includes')
   includes = set()
   if includes_node != None:
     includes = set(GetResourcesForNode(includes_node, grd_file, 'include'))
   return messages.union(includes)
 
 
 def GetOutputFileForNode(node):
   """Find the output file starting from a given node.
 
   Args:
@@ skipping 76 matching lines @@
     return 1
   grd_file = argv[1]
   dirs_to_scan = argv[2:]
   for f in dirs_to_scan:
     if not os.path.exists(f):
       print 'Error: %s does not exist' % f
       return 1
 
   tree = xml.etree.ElementTree.parse(grd_file)
   grit_header = GetOutputHeaderFile(tree)
+  if not grit_header:
+    print 'Error: %s does not generate any output headers.' % grit_header
+    return 1
   resources = GetResourcesForGrdFile(tree, grd_file)
 
   files_with_unneeded_grit_includes = []
   for dir_to_scan in dirs_to_scan:
     for root, dirs, files in os.walk(dir_to_scan):
       if '.git' in dirs:
         dirs.remove('.git')
       full_paths = [os.path.join(root, f) for f in files if ShouldScanFile(f)]
       files_with_unneeded_grit_includes.extend(
           [f for f in full_paths
            if not NeedsGritInclude(grit_header, resources, f)])
   if files_with_unneeded_grit_includes:
     print '\n'.join(files_with_unneeded_grit_includes)
     return 2
   return 0
 
 
 if __name__ == '__main__':
   sys.exit(main(sys.argv))