Fail successive scan() invocations on readers

Signed-off-by: Zoltan Kis <[email protected]>

Author: zolkis

index.html

@@ -3474,7 +3474,12 @@ <h3><dfn>Writing content</dfn></h3>
                 and abort these steps.
               </li>
               <li>
-                Add |reader| to the <a>activated reader objects</a>.
+                If |reader| has already initiated a scan, then reject |p| with a
+                {{"InvalidStateError"}} {{DOMException}} and abort these steps.
+              </li>
+              <li>
+                Add |reader| to the <a>activated reader objects</a> and mark
+                that it has initiated a scan.
               </li>
               <li>
                 Resolve |p|.