Add beforeRegister to the Polymer externs. This is defined in the doc…

…s here: https://www.polymer-project.org/1.0/docs/devguide/registering-elements#registration-callback

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=218450697

contrib/externs/polymer-1.0.js

@@ -123,6 +123,8 @@ PolymerElement.prototype.observers;
 PolymerElement.prototype.created = function() {};
 /** On ready callback. */
 PolymerElement.prototype.ready = function() {};
+/** On before register callback. */
+PolymerElement.prototype.beforeRegister = function() {};
 /** On registered callback. */
 PolymerElement.prototype.registered = function() {};
 /** On attached to the DOM callback. */