Only break for 101 in "websocket" mode

Also document how we should refactor this in the future.

Tests: https://wpt.fyi/fetch/security/1xx-response.any.html.

Fixes #1397.

Author: annevk

fetch.bs

@@ -5543,9 +5543,14 @@ optional boolean <var>forceNewConnection</var> (default false), run these steps:
 
        <li>
         <p>If <var>status</var> is in the range 100 to 199, inclusive:
+        <!-- If there comes a time when there is another code besides 101 and 103 that needs special
+        processing we should generalize the "process early hints response" callback and invoke it
+        for each 1xx response and as such require callers like WebSockets to do the filtering and
+        processing we now perform here inline. -->
 
         <ol>
-         <li><p>If <var>status</var> is 101, <a for=iteration>break</a>.
+         <li><p>If <var>request</var>'s <a for=request>mode</a> is "<code>websocket</code>" and
+         <var>status</var> is 101, then <a for=iteration>break</a>.
 
          <li><p>If <var>status</var> is 103 and <var>fetchParams</var>'s
          <a for="fetch params">process early hints response</a> is non-null, then