Drop nullable document.domain experiment

This reverts https://crrev.com/7a5c7e067f550762 to retire the experiment
to allow `document.domain = null`.

Bug: 733150
Change-Id: I29304d26aafe90c5a6e0d5dc15704718772fc522
Reviewed-on: https://chromium-review.googlesource.com/c/1446265
Reviewed-by: Mike West <mkwst@chromium.org>
Commit-Queue: Taiju Tsuiki <tzik@chromium.org>
Cr-Commit-Position: refs/heads/master@{#630218}
6 files changed