Password manager: Remove references to BrowserThread in core unit tests.

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

BUG=342313

Review URL: https://codereview.chromium.org/159033002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@250445 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 250445