Fix issue to get accurate iframe video src (#573)

Author: franfaccin

src/features/click-to-load.js

@@ -523,7 +523,7 @@ function replaceTrackingElement (widget, trackingElement, placeholderElement) {
     // from the DOM after they are collapsed. As a workaround, have the iframe
     // load a benign data URI, so that it's uncollapsed, before removing it from
     // the DOM. See https://crbug.com/1428971
-    const originalSrc = elementToReplace.src
+    const originalSrc = elementToReplace.src || elementToReplace.getAttribute('data-src')
     elementToReplace.src =
         'data:text/plain;charset=utf-8;base64,' + btoa('https://crbug.com/1428971')