DevTools: Nuke Timeline extension api

Timeline extension API is removed from DevTools front-end.

BUG=403684
R=pfeldman@chromium.org

Review URL: https://codereview.chromium.org/477103002

git-svn-id: svn://svn.chromium.org/blink/trunk@180350 bbb929c8-8fbe-4397-9dbb-9b2b20218538