Processing the manifest: Simplify the interface.

Replaces the link and response parameters with document URL and manifest URL.

These parameters were only used to get the document URL and manifest
URL, respectively, so it doesn't make sense to accept the much bulkier
HTML objects. This was limiting the ability to call the processing
algorithm from outside an HTML document context, which is a future
direction we wish to explore.

Note that the only call to this algorithm is in the HTML spec, which
needs to be updated simultaneously to use the new interface.

Pre-work for w3c#668.

index.html

@@ -1254,15 +1254,10 @@ <h3>
           <p>
             The steps for <dfn data-export="" data-local-lt=
             "processing">processing a manifest</dfn> are given by the following
-            algorithm. The algorithm takes [^link^] |el:HTMLLinkElement|, a
-            [=Response=] |response|, and a [=byte sequence=] |bodyBytes|.
+            algorithm. The algorithm takes a [=URL=] |document URL:URL|, a
+            [=URL=] |manifest URL:URL|, and a [=byte sequence=] |bodyBytes|.
           </p>
           <ol class="algorithm">
-            <li>Let |document URL:URL| be |el|'s [=Node/node document=]'s
-            [=Document/URL=].
-            </li>
-            <li>Assert: |document URL:URL| is not null.
-            </li>
             <li>Let |json| be the result of [=parse JSON bytes to an Infra
             value=] passing |bodyBytes|.
             </li>
@@ -1285,8 +1280,6 @@ <h3>
             <li>[=Process a text member=] passing |json|, |manifest|, and
             "short_name".
             </li>
-            <li>Let |manifest URL:URL| be |response|'s [=response/URL=].
-            </li>
             <li>[=Process the `start_url` member=] passing |json|, |manifest|,
             |manifest URL|, and |document URL|.
             </li>