Add origin as query string to requests to plotlyServerURL

Author: marthacryan

src/plots/plots.js

@@ -213,7 +213,12 @@ plots.sendDataToCloud = function(gd, serverURL) {
 
     // Open the Cloud login page in a new tab. We keep a reference so we can post
     // the chart back to it once Cloud reports that authentication succeeded.
-    var cloudWindow = window.open(serverURL, '_blank');
+    // Pass the current page's origin as a query string so Cloud knows where to
+    // send the CHART_AUTH_SUCCESS message back to.
+    var uploadUrl = serverURL +
+        (serverURL.indexOf('?') === -1 ? '?' : '&') +
+        'origin=' + encodeURIComponent(window.location.origin);
+    var cloudWindow = window.open(uploadUrl, '_blank');
     if(!cloudWindow) {
         console.error('Unable to open Plotly Cloud (the popup may have been blocked)');
         gd.emit('plotly_exportfail');