[CRX] Stop intercepting ftp in Chrome 59+

The extension cannot easily fetch data from ftp:-resources any more
in Chrome 59+. So don't intercept such URLs.

extensions/chromium/pdfHandler.js

@@ -153,10 +153,16 @@ chrome.webRequest.onBeforeRequest.addListener(
     urls: [
       'file://*/*.pdf',
       'file://*/*.PDF',
+      ...(
+        // Duck-typing: MediaError.prototype.message was added in Chrome 59.
+        MediaError.prototype.hasOwnProperty('message') ? [] :
+        [
           // Note: Chrome 59 has disabled ftp resource loading by default:
           // https://www.chromestatus.com/feature/5709390967472128
           'ftp://*/*.pdf',
           'ftp://*/*.PDF',
+        ]
+      ),
     ],
     types: ['main_frame', 'sub_frame'],
   },