Introduce "is offline" algorithm and use it

This is needed for WebDriver BiDi emulated network offline and allows navigator.onLine to build upon the same primitive.

Tests: web-platform-tests/wpt#55300.

HTML PR: whatwg/html#11575.

Author: sadym-chromium

fetch.bs

@@ -412,6 +412,17 @@ steps:
 </ol>
 </div>
 
+<div algorithm>
+<p>To check if the <a>environment settings object</a> <var>environment</var>
+<dfn export>is offline</dfn>:
+
+<ul>
+ <li><p>If the user agent assumes it does not have internet connectivity, then return true.
+
+ <li><p>Return <var>environment</var>'s <a>WebDriver BiDi network is offline</a>.
+</ul>
+</div>
+
 <hr>
 
 <p>To <dfn>serialize an integer</dfn>, represent it as a string of the shortest possible decimal
@@ -6391,6 +6402,9 @@ optional boolean <var>forceNewConnection</var> (default false), run these steps:
 <ol>
  <li><p>Let <var>request</var> be <var>fetchParams</var>'s <a for="fetch params">request</a>.
 
+ <li><p>If <var>request</var>'s <a for=request>client</a> <a>is offline</a>, then return a
+ <a>network error</a>.
+
  <li><p>Let <var>response</var> be null.
 
  <li><p>Let <var>timingInfo</var> be <var>fetchParams</var>'s <a for="fetch params">timing info</a>.