Add a render-blocking boolean flag to the request concept

The HTML-side counterpart is whatwg/html#7885.

Fixes #1433.

fetch.bs

@@ -1839,6 +1839,11 @@ Unless stated otherwise, it is false.
 
 <p class=note>This is for exclusive use by HTML's navigate algorithm. [[!HTML]]
 
+<p>A <a for=/>request</a> has an associated boolean <dfn export for=request>render-blocking</dfn>.
+Unless stated otherwise, it is false.
+
+<p class=note>This flag is for exclusive use by HTML's render-blocking mechanism. [[!HTML]]
+
 <hr>
 
 <p>A <a for=/>request</a> has an associated
@@ -3991,8 +3996,9 @@ the request.
 
  <li>
   <p>If <var>request</var>'s <a for=request>priority</a> is null, then use <var>request</var>'s
-  <a for=request>initiator</a> and <a for=request>destination</a> appropriately in setting
-  <var>request</var>'s <a for=request>priority</a> to a user-agent-defined object.
+  <a for=request>initiator</a>, <a for=request>destination</a>, and
+  <a for=request>render-blocking</a> appropriately in setting <var>request</var>'s
+  <a for=request>priority</a> to a user-agent-defined object.
 
   <p class=note>The user-agent-defined object could encompass stream weight and dependency for
   HTTP/2, and equivalent information used to prioritize dispatch and processing of HTTP/1 fetches.