[Mac] Fix flaky crash when locking a profile.

The crash was happening because the ProfileChooserController's -dealloc
was called after its Browser pointer was cleaned up, which was bad news bears.
(It attempted to remove the Oauth2TokenService observer using a corrupted
profile ptr).

Also fixed an incorrect member access in the UserManagerScreenHandler where a member was accessed possibly after its 'this' pointer went away.

BUG=337875

Review URL: https://codereview.chromium.org/150113004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@248087 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 248087