[cleanup] Remove Length::Value() and Length::GetFloatValue().

Remove an old TODO in the codebase (and fix a few small errors along the
way).

Convert most callsites to either Pixels()/Percent()/Flex(), and protect
with an additional DCHECK() if required.

Removes the test: StyleEngineTest.ViewportDescription As this wasn't
actually testing anything. It was relying on `Length::Value() == 0f` for
keyword values, and was basically just asserting that `0f == 3.5f * 0f`
which isn't useful.

Bug: 41142630
Change-Id: I78b9e97fecb4324561de604a36b2e139c1bb4664
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6360927
Commit-Queue: Ian Kilpatrick <ikilpatrick@chromium.org>
Reviewed-by: Morten Stenshorne <mstensho@chromium.org>
Reviewed-by: Koji Ishii <kojii@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1434203}