Implemented SystemUse24HourClockPolicyTest browser test.

Fixed bug in chromeos::SystemTrayDelegate - added PropertyChangeComplete()
notification.

Moved chromeos::SystemTrayDelegate to separate header file and renamed to
chromeos::SystemTrayDelegateChromeOS (from anonymous namespace).

Moved DateDefaultView to separate header.

BUG=269802
TEST=browsertest

Review URL: https://codereview.chromium.org/132383004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@250101 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 250101