Bug 1465592 - Enable Shadow DOM unconditionally in chrome documents;r=smaug

Differential Revision: https://phabricator.services.mozilla.com/D6278

--HG--
extra : moz-landing-system : lando

dom/base/Element.cpp

@@ -1182,7 +1182,8 @@ Element::AttachShadow(const ShadowRootInit& aInit, ErrorResult& aError)
    * 1. If context object’s namespace is not the HTML namespace,
    *    then throw a "NotSupportedError" DOMException.
    */
-  if (!IsHTMLElement()) {
+  if (!IsHTMLElement() &&
+      !(XRE_IsParentProcess() && IsXULElement() && nsContentUtils::AllowXULXBLForPrincipal(NodePrincipal()))) {
     aError.Throw(NS_ERROR_DOM_NOT_SUPPORTED_ERR);
     return nullptr;
   }

dom/base/nsDocument.cpp

@@ -2163,7 +2163,8 @@ nsDocument::Init()
 
   // Set this when document is initialized and value stays the same for the
   // lifetime of the document.
-  mIsShadowDOMEnabled = nsContentUtils::IsShadowDOMEnabled();
+  mIsShadowDOMEnabled = nsContentUtils::IsShadowDOMEnabled() ||
+    (XRE_IsParentProcess() && AllowXULXBL());
 
   // If after creation the owner js global is not set for a document
   // we use the default compartment for this document, instead of creating

dom/tests/mochitest/webcomponents/test_xul_custom_element.xul

@@ -31,6 +31,8 @@
     class TestCustomElement extends XULElement {
       constructor() {
         super();
+
+        this.attachShadow({mode: "open"});
       }
 
       connectedCallback() {
@@ -87,6 +89,7 @@
 
     function basicCustomElementCreate() {
       let element = document.createElementNS(XUL_NS, "test-custom-element");
+      ok(element.shadowRoot, "Shadow DOM works even with pref off");
       document.querySelector("#content").appendChild(element);
       is(element.textContent, "foo", "Should have set the textContent");
       ok(element instanceof TestCustomElement, "Should be an instance of TestCustomElement");