Passwords: Remove references to BrowserThread from PasswordStore.

Inject a TaskRunner into the PasswordStore on creation, rather than
directly referencing the UI thread.

BUG=335136
R=gcasto@chromium.org

Review URL: https://codereview.chromium.org/152683002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@248729 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 248729