Run "Prepare to run script" to initialize the JS for screencast recording in the specification execution environment.

Author: lutien

index.bs

@@ -5191,10 +5191,15 @@ The [=remote end steps=] with |command parameters| are:
 1. If the implementation is unable to record a screencast of |navigable| for any
    reason then return [=error=] with [=error code=] [=unsupported operation=].
 
+1. Let |environment settings| be the [=environment settings object=] representing
+   a specification execution environment.
+
+1. [=Prepare to run script=] with |environment settings|.
+
 1. Let |promise| be the result of [=trying=] to [=capture a browsing context viewport=] with
    |navigable| and |command parameters|["<code>streamOptions</code>"].
 
-1. [=React=] to promise:
+1. [=React=] to |promise|:
 
    1. If |promise| was rejected, return [=error=] with [=error code=] [=unknown error=].
 
@@ -5235,6 +5240,8 @@ The [=remote end steps=] with |command parameters| are:
 
       1. [=Call=]({{MediaRecorder/start}}, |media recorder|, |command parameters|["<code>timeslice</code>"]).
 
+      1. [=Clean up after running script=] with |environment settings|.
+
       1. Set [=screencast recordings map=][|screencast|] to |recording|.
 
       1. Return a new [=/map=] matching the <code>browsingContext.StartScreencastResult</code>