Enable Enterprise enrollment on desktop builds.

Enable Enterprise enrollment on desktop builds.

Note: this is a reland of https://codereview.chromium.org/258743005

This change implements some of the DBus stub methods so that enterprise enrollment works on desktop builds. That will make development of features that depend on enrollment faster for developers that use this workflow (e.g. for kiosk enterprise apps, public accounts, testing some device policies, etc).

- Override some of the directories and files involved with the enrollment state
- Simple stub implementation of the DBus calls involved
- Write a persistent cache of the install attributes
- Cleaned up the stub for user cloud policy and made them persistent too
- Updated some tests

This change doesn't affect production code.

TBR=jochen@chromium.org
BUG=240269, 367674

Review URL: https://codereview.chromium.org/264943011

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@267861 0039d316-1c4b-4281-b951-d872f2087c98