Misc cros_browser_backend cleanup
* _IsLoggedIn checks that the browser devtools api is responsive.
* _WaitForGuestFsMounted replaced by _IsCryptohomeMounted.
* Get rid of some unnecessary oobe_exists checks.
* Rename _NavigateLogin to _NavigateFakeLogin
* Move checks for login and post-login activities into _WaitForLogin.
BUG=308757
TEST=manual
NOTRY=True
Review URL: https://codereview.chromium.org/
208423005
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@258789 0039d316-1c4b-4281-b951-d872f2087c98