CSS supports escaping in selectors, e.g. writing `.foo:bar` will match an element with the
`foo` class and `bar` pseudo-class, but `.foo\:bar` will match the `foo:bar` class. Our
shimmed shadow DOM encapsulation always assumes that `:` means a pseudo selector
which breaks a selector like `.foo\:bar`.
These changes add some extra logic so that escaped characters in selectors are preserved.
Fixes#31844.
PR Close#40264
335d6c8c00