Support cosmetic filtering !important display overrides in "Standard" blocking mode

[antonok-edm]

#22264 added support for most types of cosmetic filtering to override !important in-page style properties. It is still not yet supported in Standard shields mode due to the use of document.adoptedStyleSheets. The addRule and removeRule APIs it provides are required to support protecting 1st-party page content, but this injection method but will fail to override an in-page style with a higher specificity if it has !important. document.adoptedStyleSheets is injected as an author-origin style at the end of the page's stylesheets.

All other cosmetic filter CSS injection uses WebDocument::InsertStyleSheet, which inserts CSS as a user-origin style. This is ideal, other than the fact that there is no equivalent to the addRule and removeRule APIs for it. There is WebDocument::RemoveInsertedStyleSheet, but building the equivalent injection system with one stylesheet per rule has prohibitive performance impact.

The ideal solution will involve exposing the addRule and removeRule methods on the C++ CSSStyleSheet class so it can be used for user-origin stylesheets, but through a new public API in Blink code (since CSSStyleSheet is not an exported member).

[ryanbr]

Safe to close @antonok-edm ?