Part 5 of search provider refactoring.

Factor out a shared Stop() method into superclass. The method is
public because AutocompleteController calls Stop() on
zero_suggest_provider. There is no change to functionality.

BUG=338955
TBR=mpearson

Review URL: https://codereview.chromium.org/158223002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@251746 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 251746