Fix several tests that were broken for CrOS multi-profiles

* Make all such tests running with and w/o --multi-profiles on Chrome OS

Infrastructure fixes:
* Setup kGoogleServicesUsername pref for stub user
* ScopedTestUserManager: initialize ProfileHelper so that it subscribes to active user change (keeps track of hash change)

Test fixes:
* LoginUserTest: user proper profile path
* LoginUtilsTest (not a real browser test): initialize MessageCenter, don't pass --login-profile in case of multi-profiles
* PowerPolicyBrowserTestBase: use active user profile
* ProfileManagerGuestTest: proper value for --login-profile
* PrefsTabHelperBrowserTest: user proper profile path
* SessionCrashedInfoBarDelegateUnitTest: properly initialize register LocalState

* BrowserCommandControllerTest, AvatarMenuButtonTest, NewAvatarMenuButtonTest: profile list menu should be disabled on CrOS

BUG=351303
R=mnissler@chromium.org, msw@chromium.org, noamsml@chromium.org, noms@chromium.org, rogerta@chromium.org, xiyuan@chromium.org

Review URL: https://codereview.chromium.org/194973002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@256806 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 256806