Add environment's execution ready flag

Service worker's Client's reserved attribute and clients.matchAll(/* options.includeReserved */) method require getting the state of a Client object whether its environment is in reserved state or not.

This commit adds the execution ready flag to the environment concept that distinguishes the state between before and after the user agent is ready to parse a fetched (or given default) document or execute a worker script.

The execution ready flag is set when the global object and realm execution context are created and the main resource (for parsing/executing) is ready.

Related service worker issue: w3c/ServiceWorker#1003.

Fixes whatwg#1992.

source

@@ -76637,8 +76637,8 @@ dictionary <dfn>DragEventInit</dfn> : <span>MouseEventInit</span> {
     </ul>
    </li>
 
-   <li><p><span>Set up a browsing context environment settings object</span> with
-   <var>realm execution context</var>.</p></li>
+   <li><p><span>Set up a browsing context environment settings object</span> with <var>realm
+   execution context</var>, and let <var>settingsObject</var> be the result.</p></li>
 
    <li><p>Let <var>document</var> be a new <code>Document</code>, marked as an <span data-x="HTML
    documents">HTML document</span> in <span>quirks mode</span>, whose <span
@@ -76682,6 +76682,9 @@ dictionary <dfn>DragEventInit</dfn> : <span>MouseEventInit</span> {
 
    <li><p><span>Implement the sandboxing</span> for <var>document</var>.</p></li>
 
+   <li><p>Set <var>settingsObject</var>'s <span
+   data-x="concept-environment-execution-ready-flag">execution ready flag</span>.</p></li>
+
    <li><p>Add <var>document</var> to <var>browsingContext</var>'s <span>session
    history</span>.</p></li>
 
@@ -81931,6 +81934,9 @@ State: &lt;OUTPUT NAME=I>1&lt;/OUTPUT> &lt;INPUT VALUE="Increment" TYPE=BUTTON O
 
      <li><p><span>Implement the sandboxing</span> for the <code>Document</code>.</p></li>
 
+     <li><p>Set <var>settingsObject</var>'s <span
+     data-x="concept-environment-execution-ready-flag">execution ready flag</span>.</p></li>
+
      <li>
       <p>If <var>settingsObject</var>'s <span
       data-x="concept-environment-active-service-worker">active service worker</span> is not null,
@@ -85934,6 +85940,11 @@ interface <dfn>NavigatorOnLine</dfn> {
    data-dfn-for="environment">active service worker</dfn></dt>
    <dd><p>Null or a <span data-x="dfn-service-worker">service worker</span> that <span
    data-x="dfn-control">controls</span> the <span>environment</span>.</p></dd>
+
+   <dt>An <dfn data-x="concept-environment-execution-ready-flag" data-export=""
+   data-dfn-for="environment">execution ready flag</dfn></dt>
+   <dd><p>A flag that indicates whether the environment setup is done. It is initially
+   unset.</p></dd>
   </dl>
 
   <p>An <dfn data-export="">environment settings object</dfn> is an <span>environment</span> that
@@ -96497,6 +96508,9 @@ interface <dfn>SharedWorkerGlobalScope</dfn> : <span>WorkerGlobalScope</span> {
 
    </li>
 
+   <li><p>Set <var>inside settings</var>'s <span
+   data-x="concept-environment-execution-ready-flag">execution ready flag</span>.</p></li>
+
    <!-- SCRIPT EXEC -->
 
    <li>