Add ShadowRoot clonable attribute

Fixes #1236.

Author: saschanaz

dom.bs

@@ -5859,6 +5859,7 @@ interface ShadowRoot : DocumentFragment {
   readonly attribute ShadowRootMode mode;
   readonly attribute boolean delegatesFocus;
   readonly attribute SlotAssignmentMode slotAssignment;
+  readonly attribute boolean clonable;
   readonly attribute Element host;
   attribute EventHandler onslotchange;
 };
@@ -5907,6 +5908,9 @@ null if <var>event</var>'s <a>composed flag</a> is unset and <a for=/>shadow roo
 <p>The <dfn attribute for=ShadowRoot><code>slotAssignment</code></dfn> getter steps are to return
 <a>this</a>'s <a for=ShadowRoot>slot assignment</a>.
 
+<p>The <dfn attribute for=ShadowRoot><code>clonable</code></dfn> getter steps are to return
+<a>this</a>'s <a for=ShadowRoot>clonable</a>.
+
 <p>The <dfn attribute for=ShadowRoot><code>host</code></dfn> getter steps are to return
 <a>this</a>'s <a for=DocumentFragment>host</a>.