Do not keep service workers alive via prefetch clients

Fixes #347: the "clients" created during prefetch time no longer count as controlling pages, in a way that prevents service worker updates. We may still want to consider evicting prefetch records on service worker updates (as discussed in #351), but that can be done as an additional step on top of this.

Author: domenic

prefetch.bs

@@ -779,6 +779,16 @@ This section contains patches to [[NAVIGATION-TIMING]].
 
 This section contains patches to [[SERVICE-WORKERS]].
 
+<div algorithm="Service worker control">
+  Modify the definition of <a spec="SERVICE-WORKERS">controlled</a> as follows:
+
+  <blockquote>
+    When a [=service worker client=] has a non-null [=environment/active service worker=]<ins> whose [=environment/is navigational prefetch client=] is false</ins>, it is said to be <dfn noexport>controlled</dfn> by that [=environment/active service worker=].
+  </blockquote>
+</div>
+
+<hr>
+
 <div algorithm="Create Fetch Event and Dispatch">
   Modify <a spec="SERVICE-WORKERS">Create Fetch Event and Dispatch</a>'s step which sets {{FetchEvent/resultingClientId}} as follows: