ScrollTimeline: make endScrollOffset inclusive only when max-scroll-position

Before this CL, we always treated endScrollOffset as inclusive, against
the spec. The spec was recently updated to treat endScrollOffset as
inclusive but only when it is equal to max-offset:
https://github.com/WICG/scroll-animations/pull/37

Bug: 934989
Change-Id: I7cd9cf4619e804a54cef33ef8d3ec7395166bace
Reviewed-on: https://chromium-review.googlesource.com/c/1483682
Commit-Queue: Stephen McGruer <smcgruer@chromium.org>
Reviewed-by: Majid Valipour <majidvp@chromium.org>
Cr-Commit-Position: refs/heads/master@{#636673}
5 files changed