Remove PropertyRegistration::Create.

This was added recently for testing purposes, but I wasn't aware of
FRIEND_TEST_ALL_PREFIXES at the time. PropertyRegistration instances must
never exist unless CSSVariables2 is enabled, and I've been worried this
might accidentally happen ever since ::Create was added.

Added bonus DCHECK for even more inner peace.

R=futhark@chromium.org

Bug: 641877
Change-Id: I61780ec25fb09673a0f3325775ce1b891b0867cb
Reviewed-on: https://chromium-review.googlesource.com/1218702
Reviewed-by: Rune Lillesveen <futhark@chromium.org>
Commit-Queue: Anders Ruud <andruud@chromium.org>
Cr-Commit-Position: refs/heads/master@{#590267}
3 files changed