Mark Notifications::show() as being part of the legacy API.

This method is not part of the latest Web Notification API drafts. Guard
it behind the ENABLE(LEGACY_NOTIFICATIONS) compile-time flag for clarity.

BUG=

Review URL: https://codereview.chromium.org/68103019

git-svn-id: svn://svn.chromium.org/blink/trunk@162033 bbb929c8-8fbe-4397-9dbb-9b2b20218538