Change URL's port attribute's setter

When it's setter is set to the empty string, it should set the underlying URL's port to null. Previously that was a no-op.

This matches Firefox. (Chromium sets the port to zero.)

Test: web-platform-tests/wpt#4101

Demo: http://software.hixie.ch/utilities/js/live-dom-viewer/saved/4054

url.bs

@@ -2637,8 +2637,12 @@ does return a <a for=urlsyntax>port</a> so one might have assumed the setter to
  <a>cannot-be-a-base-URL flag</a> is set, or its <a for=url>scheme</a> is "<code>file</code>",
  terminate these steps.
 
- <li><p><a lt="basic URL parser">Basic URL parse</a> the given value with <a>context object</a>'s
- <a for=URL>url</a> as <var>url</var> and <a>port state</a> as <var>state override</var>.
+ <li><p>If the given value is the empty string, then set <a for=URL>url</a>'s <a for=url>port</a> to
+ null.</p></li>
+
+ <li><p>Otherwise, <a lt="basic URL parser">basic URL parse</a> the given value with
+ <a>context object</a>'s <a for=URL>url</a> as <var>url</var> and <a>port state</a> as
+ <var>state override</var>.
 </ol>
 
 <p>The <dfn attribute for=URL><code>pathname</code></dfn> attribute's getter must run these steps:

url.html

@@ -1945,7 +1945,10 @@ <h3 class="heading settled" data-level="6.2" id="urlutils-members"><span class="
      <p>If <a data-link-type="dfn" href="https://dom.spec.whatwg.org/#context-object">context object</a>’s <a data-link-type="dfn" href="#concept-url-url">url</a>’s <a data-link-type="dfn" href="#concept-url-host">host</a> is null, its <a data-link-type="dfn" href="#url-cannot-be-a-base-url-flag">cannot-be-a-base-URL flag</a> is set, or its <a data-link-type="dfn" href="#concept-url-scheme">scheme</a> is "<code>file</code>",
  terminate these steps. </p>
     <li>
-     <p><a data-link-type="dfn" href="#concept-basic-url-parser">Basic URL parse</a> the given value with <a data-link-type="dfn" href="https://dom.spec.whatwg.org/#context-object">context object</a>’s <a data-link-type="dfn" href="#concept-url-url">url</a> as <var>url</var> and <a data-link-type="dfn" href="#port-state">port state</a> as <var>state override</var>. </p>
+     <p>If the given value is the empty string, then set <a data-link-type="dfn" href="#concept-url-url">url</a>’s <a data-link-type="dfn" href="#concept-url-port">port</a> to
+ null.</p>
+    <li>
+     <p>Otherwise, <a data-link-type="dfn" href="#concept-basic-url-parser">basic URL parse</a> the given value with <a data-link-type="dfn" href="https://dom.spec.whatwg.org/#context-object">context object</a>’s <a data-link-type="dfn" href="#concept-url-url">url</a> as <var>url</var> and <a data-link-type="dfn" href="#port-state">port state</a> as <var>state override</var>. </p>
    </ol>
    <p>The <dfn class="idl-code" data-dfn-for="URL" data-dfn-type="attribute" data-export="" id="dom-url-pathname"><code>pathname</code><a class="self-link" href="#dom-url-pathname"></a></dfn> attribute’s getter must run these steps: </p>
    <ol>