Remove window.performance.webkitGet*

Intent to Remove:
https://groups.google.com/a/chromium.org/d/msg/blink-dev/ItNwgL2Mv_E/AgPiVBz1gUwJ

BUG=366325

Review URL: https://codereview.chromium.org/253353002

git-svn-id: svn://svn.chromium.org/blink/trunk@172683 bbb929c8-8fbe-4397-9dbb-9b2b20218538