GitLab

This CL adds several changes:
- enables the JS handler to turn on and off the hotword recognizer
- adds a pref 'hotword.app_list_enabled' to customize the plugin usage
- synchronization logic, so if the user explicitly turns off
  hotword.search_enabled, hotword.app_list_enabled goes off too
- extract the language check of HotwordService as a static method
  and let StartPageHandler check it, because our hotword
  "Ok, Google" is US-English only

BUG=341710, 341779
R=xiyuan@chromium.org, rlp@chromium.org, estade@chromium.org
TBR=samarth@chromium.org
TEST=manually

Review URL: https://codereview.chromium.org/158143002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@251142 0039d316-1c4b-4281-b951-d872f2087c98