Add a user preference for Contextual Search.

BUG=341761

Review URL: https://codereview.chromium.org/160053004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@250920 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 250920
[Hotword] Adding the hotword action I forgot to add in https://codereview.chromium.org/153403004/

BUG=325439, 289023

Review URL: https://codereview.chromium.org/148003004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@250921 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 250921