Move password store unit tests and MockPasswordStore into component.

This CL depends on https://codereview.chromium.org/160463002/.

BUG=334665
R=blundell@chromium.org
TBR=joi@chromium.org, zea@chromium.org

Review URL: https://codereview.chromium.org/148523017

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@251672 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 251672