WebDriver BiDi: emulate default User-Agent value

Corresponding Fetch PR: whatwg/fetch#1851.

Author: sadym-chromium

source

@@ -2603,7 +2603,7 @@ a.setAttribute('href', 'https://example.com/'); // change the content attribute
      <li>A <dfn data-x-href="https://fetch.spec.whatwg.org/#local-scheme">local scheme</dfn></li>
      <li>A <dfn data-x-href="https://fetch.spec.whatwg.org/#fetch-scheme">fetch scheme</dfn></li>
      <li><dfn data-x-href="https://fetch.spec.whatwg.org/#http-cors-protocol">CORS protocol</dfn></li>
-     <li><dfn data-x="default-user-agent-value" data-x-href="https://fetch.spec.whatwg.org/#default-user-agent-value">default `<code>User-Agent</code>` value</dfn></li>
+     <li><dfn data-x="environment-default-user-agent-value" data-x-href="https://fetch.spec.whatwg.org/#environment-default-user-agent-value">environment default `<code>User-Agent</code>` value</dfn></li>
      <li><dfn data-x-href="https://fetch.spec.whatwg.org/#concept-header-extract-mime-type">extract a MIME type</dfn></li>
      <li><dfn data-x-href="https://fetch.spec.whatwg.org/#legacy-extract-an-encoding">legacy extract an encoding</dfn></li>
      <li><dfn data-x="concept-fetch" data-x-href="https://fetch.spec.whatwg.org/#concept-fetch">fetch</dfn></li>
@@ -121071,12 +121071,14 @@ interface <dfn interface>Navigator</dfn> {
   data-x="dom-navigator-appVersion">appVersion</code></dfn> getter steps are:</p>
 
   <ol>
-   <li><p>If the <span data-x="default-user-agent-value">default `<code>User-Agent</code>`
-   value</span> does not start with `<code data-x="">Mozilla/5.0 (</code>`, then return the empty
-   string.</p></li>
+   <li><p>Let <var>userAgent</var> be <span>this</span>'s <span>relevant settings object</span>'s
+   <span data-x="environment-default-user-agent-value">environment default `<code>User-Agent</code>`
+   value</span>.</p></li>
+
+   <li><p>If <var>userAgent</var> does not start with `<code data-x="">Mozilla/5.0 (</code>`, then
+   return the empty string.</p></li>
 
-   <li><p>Let <var>trail</var> be the substring of the <span
-   data-x="default-user-agent-value">default `<code>User-Agent</code>` value</span>, <span
+   <li><p>Let <var>trail</var> be the substring of <var>userAgent</var>, <span
    data-x="isomorphic decode">isomorphic decoded</span>, that follows the "<code
    data-x="">Mozilla/</code>" prefix.</p></li>
 
@@ -121126,8 +121128,8 @@ interface <dfn interface>Navigator</dfn> {
    </dl>
 
   <p>The <dfn attribute for="NavigatorID"><code data-x="dom-navigator-userAgent">userAgent</code></dfn>
-  getter steps are to return the <span data-x="default-user-agent-value">default `<code>User-Agent</code>`
-  value</span>.</p>
+  getter steps are to return <span>this</span>'s <span>relevant settings object</span>'s <span
+  data-x="environment-default-user-agent-value">environment default `<code>User-Agent</code>` value</span>.</p>
 
   <p>The <dfn attribute for="NavigatorID"><code data-x="dom-navigator-vendor">vendor</code></dfn>
   getter steps are to return the appropriate string from the following list:</p>