[CodeHealth] Add comments to cc::ScrollTimeline::ScrollDirection

cc::ScrollTimeline::ScrollDirection uses different enums from the spec
because it does not know about the writing modes. Adding comments to
reduce ambiguity.

Bug: None
Change-Id: I1f9bf443df42d52a1a5bd9960423d2fa94d4f565
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1949113
Commit-Queue: Yi Gu <yigu@chromium.org>
Reviewed-by: Majid Valipour <majidvp@chromium.org>
Cr-Commit-Position: refs/heads/master@{#721161}

cc/animation/scroll_timeline.h

@@ -22,6 +22,10 @@ class ScrollTree;
 // https://wicg.github.io/scroll-animations/#scrolltimeline-interface.
 class CC_ANIMATION_EXPORT ScrollTimeline {
  public:
+  // cc does not know about writing modes. The ScrollDirection below is
+  // converted using blink::scroll_timeline_util::ConvertOrientation which takes
+  // the spec-compliant ScrollDirection enumeration.
+  // https://drafts.csswg.org/scroll-animations/#scrolldirection-enumeration
   enum ScrollDirection {
     ScrollUp,
     ScrollDown,