[ScrollTimeline] Remove no longer necessary TODO

Upstream, the spec has changed to remove 'auto' as a value for
ScrollDirection.

TBR=majidvp@chromium.org

Bug: None
Change-Id: I5fa47e0f31c5229b5d734dc39d0717973936543e
Reviewed-on: https://chromium-review.googlesource.com/c/1347033Reviewed-by: Stephen McGruer <smcgruer@chromium.org>
Reviewed-by: Majid Valipour <majidvp@chromium.org>
Commit-Queue: Stephen McGruer <smcgruer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#610261}

third_party/blink/renderer/core/animation/scroll_timeline.cc

@@ -28,7 +28,6 @@ ActiveScrollTimelineSet& GetActiveScrollTimelineSet() {
 
 bool StringToScrollDirection(String scroll_direction,
                              ScrollTimeline::ScrollDirection& result) {
-  // TODO(smcgruer): Support 'auto' value.
   if (scroll_direction == "block") {
     result = ScrollTimeline::Block;
     return true;