Removing a DCHECK in OverscrollWindowAnimation::OnOverscrollModeChange

Proper fix is in https://codereview.chromium.org/2983883002/, but I'd rather not
land that while on leave. I will land the proper fix when I come back if this is
still not replaced with the new UI.

BUG=668296

Review-Url: https://codereview.chromium.org/2985543002
Cr-Commit-Position: refs/heads/master@{#488328}