Previously, the parent CSS stylesheet, owner node and owner CSS rule
properties were not unset when removing a sheet from a StyleSheetList.
This change moves the methods for adding and removing sheets to and
from a StyleSheetList, directly into the StyleSheetList class and
ensures they are called as required by the CSSOM specification.
If a style element belongs to a shadow tree, its CSSStyleSheet is now
added to the corresponding ShadowRoot instead of the document.
Co-authored-by: Simon Wanner <simon+git@skyrising.xyz>
This commit removes DeprecatedString's "null" state, and replaces all
its users with one of the following:
- A normal, empty DeprecatedString
- Optional<DeprecatedString>
Note that null states of DeprecatedFlyString/StringView/etc are *not*
affected by this commit. However, DeprecatedString::empty() is now
considered equal to a null StringView.
