use a public url shortener

The design doc pointed to is public on chromium.org, but the
Googler-only shortener is not.  Use a public one instead so
anyone can follow it.

BUG=

Review URL: https://codereview.chromium.org/899413002

Cr-Commit-Position: refs/heads/master@{#330325}

chrome/common/extensions/api/image_writer_private.idl

@@ -6,7 +6,7 @@
 // removable media.
 //
 // See the design doc for a detailed description of this API.
-// https://goto.google.com/image-writer-private-designdoc
+// https://goo.gl/KzMEFq
 namespace imageWriterPrivate {
   // The different stages of a write call.
   //