Remove window.performance.webkit* (PrefixedUserTiming)

Intent to Remove:
https://groups.google.com/a/chromium.org/d/msg/blink-dev/wVPFvUwq4cc/0SKFo56fIg0J

BUG=335459

Review URL: https://codereview.chromium.org/130213008

git-svn-id: svn://svn.chromium.org/blink/trunk@165295 bbb929c8-8fbe-4397-9dbb-9b2b20218538