wm: Remove a TODO from split view test

I investigated and the issue was that we use 2.f/3.f or 0.66repeating
for the original calculations, which 533.33repeating rounded to 533 in
the first pass. On swap, we update the snap ratio using 533/800 or
0.6625, then do calculations resulting in a one pixel difference.

I floated the idea of using `SplitViewController::FindClosestPositionRatio`,
after calculating the new snap ratio. However, it was not trivial and
a 1px difference on a pretty obscure feature seemed like a reasonable
tradeoff.

Test: none
Bug: none
Change-Id: I07db6783ebd807873a90444c15eb4c9419d27675
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5484588
Reviewed-by: Sophie Wen <sophiewen@chromium.org>
Commit-Queue: Sammie Quon <sammiequon@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1292143}