Restructured PlatformVerificationFlow testing.

Also added a unit test for incognito / guest mode.

BUG=chromium:328539
TEST=unit

Review URL: https://codereview.chromium.org/106373003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@242157 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 242157