Retire  CSSOMSmoothScroll flag

This actually fixes a bug that appeared in the bindings for window.
Previously window.scrollTo.length == 2 in Chrome but was 0 in
FireFox. This is detected by the cssom-view wpt test.

CSSOM View smooth scroll API shipped in M61
https://codereview.chromium.org/2938373003
https://www.chromestatus.com/features/5812155903377408

BUG=243871

Change-Id: I7e2d5b7be38fd7dfc26e633ccd171e435eba51e5
Reviewed-on: https://chromium-review.googlesource.com/c/1323694
Commit-Queue: David Bokan <bokan@chromium.org>
Reviewed-by: David Bokan <bokan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#606672}
7 files changed