createElementNS handles element name 'xmlns' correctly.

This matches the behavior of Internet Explorer and Firefox, the DOM
Level 3 Core specification, and the DOM4 specification.

http://www.w3.org/TR/2004/REC-DOM-Level-3-Core-20040407/core.html#ID-DocCrElNS
http://www.w3.org/TR/dom/#dom-document-createelementns
http://w3c.github.io/dom/#dom-document-createelementns

BUG=330946

Review URL: https://codereview.chromium.org/122083002

git-svn-id: svn://svn.chromium.org/blink/trunk@164492 bbb929c8-8fbe-4397-9dbb-9b2b20218538