Add support for writing unit tests for Mojo-backed apps/extensions APIs.

extensions/DEPS

 include_rules = [
   # Do not add Chrome dependencies. Much work went into removing them.
   "+components/url_matcher",
   "-content",
   "+content/public/common",
   "+content/public/test",
   "+crypto",
+  "+grit/content_resources.h",
   "+grit/extensions_renderer_resources.h",
   "+grit/extensions_resources.h",
+  "+mojo/public",
   "+testing",
   "+ui",
 
   # Temporarily allowed includes as part of the app shell/extensions refactor.
   #
   # NOTE: Please do not add includes without talking to the app shell team;
   # see OWNERS for this directory.
   #
   # TODO(jamescook): Remove this. http://crbug.com/392622
   "!chrome/browser/chrome_notification_types.h",
@@ skipping 25 matching lines @@
   ],
   "(simple|complex)_feature_unittest\.cc|base_feature_provider_unittest\.cc": [
     "+chrome/common/extensions/features/chrome_channel_feature_filter.h",
   ],
   "permissions_data_unittest\.cc": [
     "+chrome/common/chrome_version_info.h",
     "+chrome/common/extensions/extension_test_util.h",
     "+chrome/common/extensions/features/feature_channel.h",
   ],
 }