Remove support for legacy WebKit Notifications

All the instances of ENABLE_LEGACY_NOTIFICATIONS has been removed.

BUG=348019

Review URL: https://codereview.chromium.org/192703004

git-svn-id: svn://svn.chromium.org/blink/trunk@168875 bbb929c8-8fbe-4397-9dbb-9b2b20218538