Exploit sharing when comparing and copying groups in ComputedStyle.

third_party/WebKit/Source/build/scripts/template_expander.py

 # Copyright (C) 2013 Google Inc. All rights reserved.
 #
 # Redistribution and use in source and binary forms, with or without
 # modification, are permitted provided that the following conditions are
 # met:
 #
 #     * Redistributions of source code must retain the above copyright
 # notice, this list of conditions and the following disclaimer.
 #     * Redistributions in binary form must reproduce the above
 # copyright notice, this list of conditions and the following disclaimer
@@ skipping 18 matching lines @@
 import os
 import sys
 
 _current_dir = os.path.dirname(os.path.realpath(__file__))
 # jinja2 is in chromium's third_party directory
 # Insert at front to override system libraries, and after path[0] == script dir
 sys.path.insert(1, os.path.join(_current_dir, *([os.pardir] * 4)))
 import jinja2
 
 
-def apply_template(path_to_template, params, filters=None):
+def apply_template(path_to_template, params, filters=None, tests=None):
     dirname, basename = os.path.split(path_to_template)
     path_to_templates = os.path.join(_current_dir, 'templates')
     jinja_env = jinja2.Environment(
         loader=jinja2.FileSystemLoader([dirname, path_to_templates]),
         keep_trailing_newline=True,  # newline-terminate generated files
         lstrip_blocks=True,  # so can indent control flow tags
         trim_blocks=True)  # so don't need {%- -%} everywhere
     if filters:
         jinja_env.filters.update(filters)
+    if tests:
+        jinja_env.tests.update(tests)
     template = jinja_env.get_template(basename)
     return template.render(params)
 
 
-def use_jinja(template_file_name, filters=None):
+def use_jinja(template_file_name, filters=None, tests=None):
     def real_decorator(generator):
         def generator_internal(*args, **kwargs):
             parameters = generator(*args, **kwargs)
-            return apply_template(template_file_name, parameters, filters=filters)
+            return apply_template(template_file_name, parameters, filters=filters, tests=tests)
         generator_internal.func_name = generator.func_name
         return generator_internal
     return real_decorator