Add "is available to element internals"

This is for use with whatwg/html#5912, which
restricts access to ElementInternals's shadowRoot property for shadow
roots which were pre-existing before a custom element
upgrade/construction. See the discussion in
WICG/webcomponents#871 (comment)
for more context.

dom.bs

@@ -5717,6 +5717,9 @@ or "<code>closed</code>").</p>
 <p><a for=/>Shadow roots</a> have an associated <dfn export for=ShadowRoot>delegates focus</dfn>.
 It is initially set to false.</p>
 
+<p><a for=/>Shadow roots</a> have an associated
+<dfn export for=ShadowRoot>available to element internals</dfn>. It is initially set to false.</p>
+
 <p><a for=/>Shadow roots</a>'s associated <a for=DocumentFragment>host</a> is never null.</p>
 <!-- If we ever change this, e.g., add a ShadowRoot object constructor, that would have serious
      consequences for innerHTML. -->
@@ -6741,6 +6744,10 @@ invoked, must run these steps:
  <li><p>Set <var>shadow</var>'s <a for=ShadowRoot>delegates focus</a> to <var>init</var>'s
  {{ShadowRootInit/delegatesFocus}}.
 
+ <li><p>If <a>this</a>'s <a for=Element>custom element state</a> is "<code>precustomized</code>" or
+ "<code>custom</code>", then set <var>shadow</var>'s
+ <a for=ShadowRoot>available to element internals</a> to true.
+
  <li><p>Set <a>this</a>'s <a for=Element>shadow root</a> to <var>shadow</var>.
 
  <li><p>Return <var>shadow</var>.