CSS: Retire runtime flag for CSSTransformBox

transform-box shipped in M64
https://chromium-review.googlesource.com/759040
https://www.chromestatus.com/feature/6643927633362944

BUG=595829

Cq-Include-Trybots: luci.chromium.try:linux_layout_tests_slimming_paint_v2;master.tryserver.blink:linux_trusty_blink_rel
Change-Id: I9e9d6a14dfef08332eaaec0e5bb6fe222282f8bb
Reviewed-on: https://chromium-review.googlesource.com/1163055
Reviewed-by: Fredrik Söderquist <fs@opera.com>
Commit-Queue: Eric Willigers <ericwilligers@chromium.org>
Cr-Commit-Position: refs/heads/master@{#580776}
3 files changed