CSS: Remove support for position values with 3 parts

Intent:
https://groups.google.com/a/chromium.org/d/msg/blink-dev/oBKMVCOX1sY/BLsXXiukAgAJ

BUG=804187

Change-Id: I94e79b2b426250c521d0ebae1492571fde078f31
Reviewed-on: https://chromium-review.googlesource.com/1013459
Reviewed-by: Emil A Eklund <eae@chromium.org>
Commit-Queue: Eric Willigers <ericwilligers@chromium.org>
Cr-Commit-Position: refs/heads/master@{#550915}
31 files changed