Fix shaka.polyfill missing in externs

The generated externs did not include shaka.polyfill because we used
the wrong annotation on the class.  This fixes it.

Raised in PR #1273

Change-Id: I348064a117a7e1878b439ad8bd1ce49df56bfd39

lib/polyfill/all.js

@@ -23,7 +23,7 @@ goog.provide('shaka.polyfill.register');
  * @namespace shaka.polyfill
  * @summary A one-stop installer for all polyfills.
  * @see http://enwp.org/polyfill
- * @exportDoc
+ * @exportInterface
  */