Add an enrolling state for consumer management section in settings page.
When the enrollment is in progress, the button shows "Enrolling..." and is
disabled.
Also made BrowserOptionsHandler an observer of ConsumerManagementService, so
that the settings page is updated when the enrollment state is changed.
BUG=
chromium:353050
TEST=unit_tests
Committed:
https://crrev.com/51ffb80c42001d78b3728e9b13cac6fd57b1a6fc
Cr-Commit-Position: refs/heads/master@{#294323}