Remove unused email from install attributes and add support for AD.
Notable changes:
* Simplify attribute accessors by ensuring that attribute members
are always self-consistent.
* Drop distinction between unset and empty attributes.
* Strict enforcement of *all* attributes being identical on
re-locking.
* IsEnterpriseDevice() evaluates the device mode instead of
the (deprecated) user.
* Improve encapsulation of StubInstallAttributes by disabling
direct member access, provide high-level setters instead.
BUG=
639295,
638713
TBR=achuith (chrome/browser/chromeos/{app_mode,login,system})
TBR=reillyg (chrome/browser/extensions)
TBR=eroman (chrome/browser/net)
TBR=bauerb (chrome/browser/ui/webui)
Committed:
https://crrev.com/42d7ea0a129585044fa4461e7d581ad88e4525dc
Cr-Commit-Position: refs/heads/master@{#424994}