Allow WebDriver BiDi to override the cache mode

The primary use case here is allowing WebDriver to enforce that requests do not
use the network cache, including the CORS preflight cache. But for
future extension the this works by calling into WebDriver BiDi to get
a cache behavior for the request, which is then translated into
specific implementation on the fetch side (e.g. setting the request's
cache mode, or not using the preflight cache).

For now the only possible values of the cache behavior are "default"
and "bypass", so we only need to check for the latter value.

fetch.bs

@@ -5237,6 +5237,8 @@ these steps:
      <li>There is at least one <a for=list>item</a> in the <a>CORS-unsafe request-header names</a>
      with <var>request</var>'s <a for=request>header list</a> for which there is no
      <a>header-name cache entry match</a> using <var>request</var>.
+
+     <li><p><a>WebDriver BiDi cache behavior</a> with <var>request</var> is "<code>bypass</code>".
     </ul>
 
     <p>Then:
@@ -5615,6 +5617,9 @@ run these steps:
    <a>default `<code>User-Agent</code>` value</a>) to <var>httpRequest</var>'s
    <a for=request>header list</a>.
 
+   <li><p>If <a>WebDriver BiDi cache behavior</a> with <var>request</var> is "<code>bypass</code>, then
+   set <var>httpRequest</var>'s <a for=request>cache mode</a> to "<code>no-store</code>".
+
    <li><p>If <var>httpRequest</var>'s <a for=request>cache mode</a> is "<code>default</code>" and
    <var>httpRequest</var>'s <a for=request>header list</a> <a for="header list">contains</a>
    `<code>If-Modified-Since</code>`,
@@ -6495,8 +6500,9 @@ populates the <a>CORS-preflight cache</a> to minimize the number of these
    <li><p>If <var>max-age</var> is greater than an imposed limit on
    <a for="cache entry">max-age</a>, then set <var>max-age</var> to the imposed limit.
 
-   <li><p>If the user agent does not provide for a <a lt="CORS-preflight cache">cache</a>, then
-   return <var>response</var>.
+   <li><p>If the user agent does not provide for a <a lt="CORS-preflight cache">cache</a>,
+   or <a>WebDriver BiDi cache behavior</a> with <var>request</var> is "<code>bypass</code>",
+   then return <var>response</var>.
 
    <li><p>For each <var>method</var> in <var>methods</var> for which there is a
    <a>method cache entry match</a> using <var>request</var>, set matching entry's