Add checkdeps presubmit check. Warns on new #includes of dependencies

tools/checkdeps/cpp_checker.py

 # 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.
 
 """Checks C++ and Objective-C files for illegal includes."""
 
 import codecs
+import os
 import re
 
 from rules import Rule
 
 
 class CppChecker(object):
 
   EXTENSIONS = [
       '.h',
       '.cc',
@@ skipping 88 matching lines @@
 
         is_include, line_status, rule_type = self.CheckLine(rules, line)
         if is_include:
           last_include = line_num
         if line_status is not None:
           if len(line_status) > 0:  # Add newline to separate messages.
             line_status += '\n'
           ret_val += line_status
 
     return ret_val
+
+  @staticmethod
+  def IsCppFile(file_path):
+    """Returns True iff the given path ends in one of the extensions
+    handled by this checker.
+    """
+    return os.path.splitext(file_path)[1] in CppChecker.EXTENSIONS