SVGAnimatedString erroneously invokes Trusted Types for SVGScriptElement.className.baseVal

[lukewarlow]

SVGScriptElement can have SVGAnimatedString properties other than href and actually I believe className is the only one.

As I read the current text, Get Trusted Type compliant string will be executed with the 'SVGScriptElement href' when one tries to set SVGScriptElement.className to a plain string, but we probably just want to use the specified value in that case.

I added more tests in to cover this (as well as href on non-script elements) where we don't want to run the TT checks: https://phabricator.services.mozilla.com/D233341. Luckily, Gecko implements SVGAnimatedString for className in a separate C++ class so it's easy to make the test pass, but I don't know whether the checking the reflected attribute name is straightforward in other engines...

Chromium fails for "Assign string to SVGScriptElement.className.baseVal.", so it indeed executes the trusted type check for class name.

Originally posted by @fred-wang in #961 (comment)

[lukewarlow]

This is a spec bug and impl bug in Chromium (and WebKit but I can fix that), and we should fix this.

cc @otherdaniel for Chromium side.