Eliminate BrowserThread usage from PasswordStoreDefault.

As part of componentizing the password manager, we need to remove all
references to content/ from PasswordStoreDefault.

Depends on https://codereview.chromium.org/152683002/.

BUG=335147
R=gcasto@chromium.org

Review URL: https://codereview.chromium.org/153943002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@248964 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 248964