Some minor clean-ups in Notification.cpp

Prefix the enum values with the enumeration's name; make m_asyncRunner
a member instead of an OwnPtr since we always create (and never destroy)
it anyway.

Review URL: https://codereview.chromium.org/394233002

git-svn-id: svn://svn.chromium.org/blink/trunk@178343 bbb929c8-8fbe-4397-9dbb-9b2b20218538