Expose a mechanism for hiding internal timing info

This is needed for whatwg/html#7104
and later on for whatwg#1215.

Navigation timing reports the timing info from the HTML spec,
so it needs a mechanism to obfuscate the internals. So far that
obfuscation was internal to fetch and was done upon reporting.

fetch.bs

@@ -231,6 +231,12 @@ following <a for=struct>items</a>: [[RESOURCE-TIMING]] [[NAVIGATION-TIMING]]
  <dd>Null or a <a for=/>connection timing info</a>.
 </dl>
 
+<p>To <dfn export lt="create an opaque timing info|creating an opaque timing info">create an
+opaque timing info</dfn>, given a <a for=/>fetch timing info</a> <var>timingInfo</var>,
+ create a new <a for=/>fetch timing info</a> whose <a for="fetch timing info">start time</a> and
+ <a for="fetch timing info">post-redirect start time</a> are <var>timingInfo</var>'s
+ <a for="fetch timing info">start time</a>.
+
 <p>To <dfn>update timing info from stored response</dfn>, given a
 <a for=/>connection timing info</a> <var>timingInfo</var> and a <a for=/>response</a>
 <var>response</var>, perform the following steps:
@@ -4131,10 +4137,8 @@ steps:
   <p>If <var>response</var>'s <a for=response>timing allow passed flag</a> is not set, then:
 
   <ol>
-   <li><p>Set <var>timingInfo</var> to a new <a for=/>fetch timing info</a> whose
-   <a for="fetch timing info">start time</a> and
-   <a for="fetch timing info">post-redirect start time</a> are <var>timingInfo</var>'s
-   <a for="fetch timing info">start time</a>.
+   <li><p>Set <var>timingInfo</var> to a the result of <a>creating an opaque timing info</a> for
+   <var>timingInfo</var>.
 
    <li><p>Set <var>cacheState</var> to the empty string.
   </ol>