Send metadatachange message to parent frame in PDF viewer

When visiting `http://<VIA_DOMAIN>/<PDF_URL>` then the PDF viewer is
displayed inside an iframe. In order for the top frame rendered by the
`proxy` view to reflect the document title and PDF URL the PDF viewer
iframe needs to send the same `metadatachange` message that ViaHTML
sends.

The "real" PDF title is not available until the document has loaded, and
is often incorrect anyway. Therefore we send the filename of the PDF
extracted from the URL if available, or the domain from which the PDF
was loaded otherwise.

via/templates/pdf_viewer.html.jinja2

@@ -28,6 +28,25 @@
 
         // URL of the Hypothesis client's boot script.
         window.CLIENT_EMBED_URL = {{ client_embed_url | tojson }};
+
+        // When this PDF is hosted inside an iframe, notify the parent frame of
+        // the PDF's metadata. ViaHTML sends the same message for HTML documents.
+        //
+        // The PDF filename is used as a title because that is immediately available
+        // before the PDF has loaded.
+        if (window.parent !== window) {
+          const pdfURL = new URL(window.PDF_URL);
+          const pathSegments = pdfURL.pathname.split('/').filter(s => s !== '');
+          const filename = pathSegments.length > 0 ? pathSegments[pathSegments.length - 1] : pdfURL.hostname;
+
+          window.parent.postMessage({
+            type: 'metadatachange',
+            metadata: {
+              location: pdfURL.href,
+              title: filename
+            },
+          }, '*');
+        }
     </script>
 
     <script src="{{ static_url("via:static/js/pdfjs-init.min.js") }}"></script>