Remove support for legacy WebKit Notifications

This patch removes support for the legacy WebKit notifications from Blink,
per the following Intent to Remove thread:

https://groups.google.com/a/chromium.org/forum/#!topic/blink-dev/8vqyfHa8_dw

BUG=348019

Review URL: https://codereview.chromium.org/184733002

git-svn-id: svn://svn.chromium.org/blink/trunk@168471 bbb929c8-8fbe-4397-9dbb-9b2b20218538