Introduce transform_important_qualifiers arg to AMP_Style_Sanitizer

[westonruter]

Summary

This is cherry-picked from #6577 and is essentially amending #6528. When custom scripts are on the page, it's important that inline style rules be left as-is without being transformed into style rules. Take comment replying for example. The cancel reply link from core gets style="display:none". The style sanitizer by default converts that into a style rule :root:not(#_):not(#_):not(#_):not(#_):not(#_) .amp-wp-cb45893 {display: none} and the style attribute will be replaced with data-amp-original-style="display:none" and class="amp-wp-cb45893". When comment-reply.js then tries to show the link by removing the the display property, nothing happens because the style has been applied with a style rule instead.

This PR adds a transform_important_qualifiers arg to AMP_Style_Sanitizer which will prevent style attributes from being converted into style rules, while also preventing the transformation of any !important qualifiers into style rules with high specificity. The latter transformation is actually the reason for the former, since specificity can only be maintained if all styles are moved to style rules.

This argument is false by default so that the valid-AMP behavior of preventing !important from appearing in the page will be preserved.

This sanitizer arg will be enabled as part of the above PR or #6546.

Checklist

• My code is tested and passes existing tests.
• My code follows the Engineering Guidelines (updates are often made to the guidelines, check it out periodically).

[github-actions]

Plugin builds for 6e187e8 are ready 🛎️!

• Download development build
• Download production build

[pierlon]

LGTM.

[fellyph]

QA Passed

Twenty Twenty-one:

Twenty-Twenty:

Twenty nineteen:

Twenty Seventeen:

Twenty Sixteen:

Twenty Fifteen:

Twenty Fourteen:

Twenty Thirteen:

Twenty eleven:

Twenty ten: