Componentize PasswordGenerationManager unittest.

The only non-trivial change is constructing a TestingPrefServiceSimple manually
rather than using the TestingProfile's PrefService.

BUG=348520

Review URL: https://codereview.chromium.org/182973005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@255064 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 255064