Make content frame in VitalSource EPUB fill full width of window

This makes the VS EPUB test page more accurately reflect the real VitalSource
viewer. More importantly, it also triggers the heuristic that the Guest class
uses to decide whether to enable side-by-side mode for an iframed guest.

dev-server/documents/html/vitalsource-epub.mustache

@@ -70,7 +70,7 @@
           const styles = document.createElement('style');
           styles.innerHTML = `
             iframe {
-              width: 600px;
+              width: 100%;
               height: 400px;
               resize: both;
               overflow: auto;