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

The interactive_ui_tests and browser_tests failures have since been
addressed by CL 187803005. This is a re-land of:
    https://codereview.chromium.org/184733002/

TBR=atwilson@chromium.org, abarth@chromium.org
BUG=348019

Review URL: https://codereview.chromium.org/188433003

git-svn-id: svn://svn.chromium.org/blink/trunk@168836 bbb929c8-8fbe-4397-9dbb-9b2b20218538