Make Window.offscreenBuffering non-enumerable

This is in order to see how much enumeration contributes to the 0.18% usage:
https://groups.google.com/a/chromium.org/d/msg/blink-dev/0LWNyj-edsQ/3TQq5vDyTHoJ

BUG=438370

Review URL: https://codereview.chromium.org/777453003

git-svn-id: svn://svn.chromium.org/blink/trunk@186405 bbb929c8-8fbe-4397-9dbb-9b2b20218538
4 files changed