Replace skip-service-worker flag with service-workers mode

This allows certain fetches within service workers to trigger fetch events. It also makes interception of redirects by foreign fetch possible.

Tests: web-platform-tests/wpt#4518.

Related service worker PR: w3c/ServiceWorker#1025.

Fixes #303 and fixes #362.

Author: jakearchibald

fetch.bs

@@ -711,8 +711,27 @@ explicitly set <a for=/>request</a>'s
 this flag set are subject to additional processing requirements.
 
 <p>A <a for=/>request</a> has an associated
-<dfn export>skip-service-worker flag</dfn>. Unless stated otherwise it is
-unset.
+<dfn for=request export>service-workers mode</dfn>, that is "<code>all</code>",
+"<code>foreign</code>", or "<code>none</code>". Unless stated otherwise it is
+"<code>all</code>".
+
+<div class=note>
+ <p>This determines which service workers will receive a {{fetch!!event}} event for this fetch.
+
+ <dl>
+  <dt>"<code>all</code>"
+  <dd>Relevant local and foreign service workers will get a {{fetch!!event}} or
+   {{foreignfetch!!event}} event for this fetch.
+
+  <dt>"<code>foreign</code>"
+  <dd>Only relevant foreign service workers will get a {{foreignfetch!!event}} event for this fetch.
+  {{fetch(input)!!method}} uses this to bypass the current service worker if the global is a
+  {{ServiceWorkerGlobalScope}}.
+
+  <dt>"<code>none</code>"
+  <dd>Neither local nor foreign service workers will get events for this fetch.
+ </dd>
+</div>
 
 <p>A <a for=/>request</a> has an associated
 <dfn export for=request id=concept-request-initiator>initiator</dfn>, which is
@@ -2806,16 +2825,13 @@ optional <i>CORS flag</i> and <i>CORS-preflight flag</i>, run these steps:
  <li><p>Let <var>actualResponse</var> be null.
 
  <li>
-  <p>If <var>request</var>'s <a>skip-service-worker flag</a> is unset, then run these
+  <p>If <var>request</var>'s <a>service-workers mode</a> is not "<code>none</code>", then run these
   substeps:
 
   <ol>
    <li>
-    <p>If <var>request</var>'s <a for=request>client</a> is null or
-    <var>request</var>'s <a for=request>client</a>'s
-    <a for="environment settings object">global object</a> is not a
-    {{ServiceWorkerGlobalScope}} object, then set <var>response</var>
-    to the result of invoking <a for=/>handle fetch</a> for <var>request</var>.
+    <p>If <var>request</var>'s <a>service-workers mode</a> is "<code>all</code>", then set
+    <var>response</var> to the result of invoking <a for=/>handle fetch</a> for <var>request</var>.
     [[!HTML]] [[!SW]]
 
    <li>
@@ -2906,7 +2922,8 @@ optional <i>CORS flag</i> and <i>CORS-preflight flag</i>, run these steps:
 
    <li>
     <p>If <var>request</var>'s <a for=request>redirect mode</a> is
-    "<code>follow</code>", then set <var>request</var>'s <a>skip-service-worker flag</a>.
+    "<code>follow</code>", then set <var>request</var>'s <a>service-workers mode</a> to
+    "<code>foreign</code>".
 
     <p class="note no-backref">Redirects coming from the network (as opposed to from a service
     worker) are not to be exposed to a service worker.
@@ -5259,6 +5276,10 @@ method, must run these steps:
  <var>init</var> as arguments. If this throws an exception, reject
  <var>p</var> with it and return <var>p</var>.
 
+ <li>If <var>request</var>'s <a for=request>client</a>'s
+ <a for="environment settings object">global object</a> is a {{ServiceWorkerGlobalScope}} object,
+ set <var>request</var>'s <a>service-workers mode</a> to "<code>foreign</code>".
+
  <li><p>Let <var>responseObject</var> be a new {{Response}} object and a new associated
  {{Headers}} object whose <a for=Headers>guard</a> is
  "<code>immutable</code>".
@@ -5410,7 +5431,7 @@ therefore not shareable, a WebSocket connection is very close to identical to an
  <li><p>Let <var>request</var> be a new <a for=/>request</a>, whose
  <a for=request>url</a> is <var>requestURL</var>,
  <a for=request>client</a> is <var>client</var>,
- <a>skip-service-worker flag</a> is set,
+ <a>service-workers mode</a> is "<code>none</code>",
  <a>synchronous flag</a> is set,
  <a for=request>mode</a> is "<code>websocket</code>",
  <a for=request>credentials mode</a> is