[MDM] Fix for incognito mode menu item

The incognito restriction was correctly gating incognito tab creation when
attempted from the 'new tab' screen, but I had missed the incognito icon
at the top of the menu.

The fix was to modify the IncognitoRestriction class to manage a new type
of control (the menu icon) and to push the gating logic down into the
Controller class (where it really belongs anyway).

Change-Id: I5395f060502907fcc7d03826568af1802450ad1b
3 files changed