Fix Facebook videos if protocol isn't specified in data-href (#1411)

Embedded Facebook videos are created by having a div element in the
page that has (amongst other attributes) a data-href attribute that
specifies the video's URL. The Facebook SDK then uses that to create
the video iframe. Our "surrogate script" does the same.

It turns out that when the protocol prefix of that URL isn't specified
(e.g. the URL starts '//'), then the SDK and our surrogate script need
to take care to add the correct protocol to the URL (e.g. 'http:' or
'https:') based on the window location. Otherwise, the video fails to
load.

shared/js/content-scripts/click-to-load.js

@@ -470,10 +470,24 @@
             }
 
             for (const key of Object.keys(this.dataElements)) {
-                const attrValue = encodeURIComponent(this.dataElements[key])
-                if (attrValue) {
-                    this.clickAction.targetURL = this.clickAction.targetURL.replace(key, attrValue)
+                let attrValue = this.dataElements[key]
+
+                if (!attrValue) {
+                    continue
+                }
+
+                // The URL for Facebook videos are specified as the data-href
+                // attribute on a div, that is then used to create the iframe.
+                // Some websites omit the protocol part of the URL when doing
+                // that, which then prevents the iframe from loading correctly.
+                if (key === 'data-href' && attrValue.startsWith('//')) {
+                    attrValue = window.location.protocol + attrValue
                 }
+
+                this.clickAction.targetURL =
+                    this.clickAction.targetURL.replace(
+                        key, encodeURIComponent(attrValue)
+                    )
             }
         }