Intent to ship: Custom Elements disabledFeatures

[Edgar Chen]

Summary: It allows custom element authors to disable the ability to attach a ShadowRoot or ElementInternal on a custom element

Bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1723396

Standard: https://html.spec.whatwg.org/multipage/custom-elements.html

Platform coverage: all

Preference: dom.webcomponents.disabledFeatures.enabled

DevTools bug: N/A

Other browsers:

 - WebKit: Not implemented yet.

 - Blink: Shipped.

Web Platform Tests: https://github.com/web-platform-tests/wpt/blob/master/shadow-dom/Element-interface-attachShadow-custom-element.html

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Edgar Chen