Replace existing usage of blob URL entry's object

As suggested in whatwg/fetch#1783 (comment), this change updates an existing use of blob URL entry's object so that the "obtain a blob object" algorithm is used instead. This algorithm was added in w3c/FileAPI#201.

source

@@ -3481,8 +3481,8 @@ a.setAttribute('href', 'https://example.com/'); // change the content attribute
      <li>The concept of <dfn data-x="file-error-read">read errors</dfn></li>
      <li><dfn data-x-href="https://w3c.github.io/FileAPI/#BlobURLStore">Blob URL Store</dfn></li>
      <li><dfn data-x-href="https://w3c.github.io/FileAPI/#blob-url-entry">blob URL entry</dfn> and its
-         <dfn data-x="blob-url-entry-object" data-x-href="https://w3c.github.io/FileAPI/#blob-url-entry-object">object</dfn> and
          <dfn data-x="blob-url-entry-environment" data-x-href="https://w3c.github.io/FileAPI/#blob-url-entry-environment">environment</dfn></li>
+     <li>The <dfn data-x="blob-url-obtain-object" data-x-href="https://w3c.github.io/FileAPI/#blob-url-obtain-object">obtain a blob object</dfn> algorithm</li>
     </ul>
    </dd>
 
@@ -36782,11 +36782,24 @@ interface <dfn interface>MediaError</dfn> {
   object</span> is as follows:</p>
 
   <ol>
-   <li><p>If the algorithm was invoked with <span>media provider object</span> or a <span>URL
-   record</span> whose <span data-x="concept-url-blob-entry">blob URL entry</span> is a <span>blob
-   URL entry</span> whose <span data-x="blob-url-entry-object">object</span> is a <span>media
-   provider object</span>, then let <var>mode</var> be <i>local</i>. Otherwise, let <var>mode</var>
-   be <i>remote</i>.</p></li>
+   <li><p>Let <var>mode</var> be <i>remote</i>.</p></li>
+
+   <li>
+    <p>If the algorithm was invoked with <span>media provider object</span>, then set
+    <var>mode</var> to <i>local</i>.</p>
+
+    <p>Otherwise:</p>
+
+    <ol>
+     <li><p>Let <var>object</var> be the result of <span data-x="blob-url-obtain-object">obtaining
+     a blob object</span> using the <span>URL record</span>'s <span
+     data-x="concept-url-blob-entry">blob URL entry</span> and the <span>media element</span>'s
+     <span>node document</span>'s <span>relevant settings object</span>.</p></li>
+
+     <li><p>If <var>object</var> is a <span>media provider object</span>, then set <var>mode</var>
+     to <i>local</i>.</p></li>
+    </ol>
+   </li>
 
    <li><p>If <var>mode</var> is <i>remote</i>, then let the <var>current media resource</var> be the
    resource given by the <span>URL record</span> passed to this algorithm; otherwise, let the