Omnibox: Refactor SearchProvider for Entity Suggestions

Cleanup SearchProvider interfaces to better support entity suggestions,
which were introduced in https://codereview.chromium.org/26688004

This is a necessary step for reviving https://codereview.chromium.org/23164011/

BUG=259486

Review URL: https://codereview.chromium.org/98713006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@239793 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 239793