Remove CSSInterpolationEnvironment::GetOptionalState

We should know when we do and when we don't have a StyleResolverState,
and GetOptionalState() makes this harder to understand.

During CSSInterpolationType::MaybeConvertUnderlyingValue, we are
handling computed values only, and there should not be a need to
resolve anything.

A valid alternative would be to *always* provide a StyleResolverState,
but that's not possible since MaybeConvertUnderlyingValue is used to
set up transitions. If we *do* need extra circumstantial information,
it needs to arrive via CSSInterpolationEnvironment, as this is provided
in all cases.

There should be no behavior change: this CL just clears up some
minor confusion that arose during work on Issue 405795970.

Bug: 405795970
Change-Id: I04a9c50891b25053d5ea1c8cbb7a5ccc86bb390f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6387164
Reviewed-by: Rune Lillesveen <futhark@chromium.org>
Commit-Queue: Rune Lillesveen <futhark@chromium.org>
Auto-Submit: Anders Hartvoll Ruud <andruud@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1437460}