[Extensions] Move [Chrome]ExtensionTestNotificationObserver into extensions::

Move the ChromeExtensionTestNotificationObserver and
ExtensionTestNotificationObserver classes into the extensions namespace.

BUG=None
TBR=stevenjb@chromium.org

Review-Url: https://codereview.chromium.org/2539623002
Cr-Commit-Position: refs/heads/master@{#435537}
11 files changed