Add UIPI support for sandbox alternate desktop

Processes must initialize user32 at a lower integrity level to
enable UIPI. So, we have to drop the integrity label of the
alternate desktop to allow processes to attach to the alternate
desktop at reduced integrity levels.

Review URL: https://codereview.chromium.org/330853002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@279424 0039d316-1c4b-4281-b951-d872f2087c98
6 files changed