Add presubmit check to warn about <label> for usage.

chrome/browser/PRESUBMIT.py

 # 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.
 
 """Presubmit script for Chromium browser code.
 
 This script currently only checks HTML/CSS/JS files in resources/.
 
 See http://dev.chromium.org/developers/how-tos/depottools/presubmit-scripts
 for more details about the presubmit API built into gcl/git cl, and see
@@ skipping 17 matching lines @@
   path = input_api.os_path
   cwd = input_api.PresubmitLocalPath()
   resources = path.join(cwd, 'resources')
   webui = path.join(cwd, 'ui', 'webui')
 
   affected_files = (f.AbsoluteLocalPath() for f in input_api.AffectedFiles())
   would_affect_tests = (
       path.join(cwd, 'PRESUBMIT.py'),
       path.join(cwd, 'test_presubmit.py'),
       path.join(cwd, 'web_dev_style', 'css_checker.py'),
+      path.join(cwd, 'web_dev_style', 'html_checker.py'),
       path.join(cwd, 'web_dev_style', 'js_checker.py'),
   )
   if any(f for f in affected_files if f in would_affect_tests):
     tests = [path.join(cwd, 'test_presubmit.py')]
     results.extend(
         input_api.canned_checks.RunUnitTests(input_api, output_api, tests))
 
   import sys
   old_path = sys.path
 
   try:
     sys.path = [cwd] + old_path
-    from web_dev_style import resource_checker, css_checker, js_checker
+    from web_dev_style import (resource_checker, css_checker, html_checker,
+                               js_checker)
 
     search_dirs = (resources, webui)
     def _html_css_js_resource(p):
       return p.endswith(('.html', '.css', '.js')) and p.startswith(search_dirs)
 
     BLACKLIST = ['chrome/browser/resources/pdf/index.html',
                  'chrome/browser/resources/pdf/index.js']
     def is_resource(maybe_resource):
       return (maybe_resource.LocalPath() not in BLACKLIST and
           _html_css_js_resource(maybe_resource.AbsoluteLocalPath()))
 
     results.extend(resource_checker.ResourceChecker(
         input_api, output_api, file_filter=is_resource).RunChecks())
     results.extend(css_checker.CSSChecker(
         input_api, output_api, file_filter=is_resource).RunChecks())
+    results.extend(html_checker.HtmlChecker(
+        input_api, output_api, file_filter=is_resource).RunChecks())
     results.extend(js_checker.JSChecker(
         input_api, output_api, file_filter=is_resource).RunChecks())
   finally:
     sys.path = old_path
 
   return results