Add ManagementMode enum and GetManagementMode().
GetManagementMode() checks PolicyData to see if the device is enterprise-managed, etc. based on the management_mode field and request_token if management_mode is not set.
Also, refactor EnrollmentHandlerChromeOS to use ManagementMode.
BUG=
chromium:436026
TEST=unit_tests
Committed:
https://crrev.com/8edac011a427f0c0180c17a5437b642a3ad016c5
Cr-Commit-Position: refs/heads/master@{#306124}