add refused-to-display tips in link-preview-overlay

css/reveal.scss

@@ -1229,6 +1229,16 @@ html:-moz-full-screen-ancestor {
 		visibility: visible;
 	}
 
+	.reveal .overlay.overlay-preview.loaded .viewport-inner  {
+		position: absolute;
+		z-index: -1;
+		left: 0;
+		top: 60px;
+		width: 100%;
+		text-align: center;
+		letter-spacing: normal;
+	}
+
 	.reveal .overlay.overlay-preview.loaded .spinner {
 		opacity: 0;
 		visibility: hidden;

js/reveal.js

@@ -1500,6 +1500,9 @@
 			'<div class="spinner"></div>',
 			'<div class="viewport">',
 				'<iframe src="'+ url +'"></iframe>',
+				'<small class="viewport-inner">',
+				'This link is refused to display in a frame due to its policy',
+				'</small>',
 			'</div>'
 		].join('');