Update Custom Elements API to new names.

LayoutTests/webexposed/custom-elements-expected.txt

 Tests basic web-exposure of Custom Elements
 
 On success, you will see a series of "PASS" messages, followed by "TEST COMPLETE".
 
 
-Document.prototype.hasOwnProperty("register")
+Document.prototype.hasOwnProperty("registerElement")
 true
 
-typeof Document.prototype.register
+typeof Document.prototype.registerElement
 function
 
 document.createElement("x-a") instanceof HTMLUnknownElement
 false
 
 document.querySelector(":unresolved")
 [object HTMLDivElement]
 
 span.getAttribute("is")
 type-extension
 
 PASS successfullyParsed is true
 
 TEST COMPLETE