Convert qname with prefix to upper-case in tagName if the context object is in the HTML namespace and its node document
is as well:
http://dom.spec.whatwg.org/#dom-element-tagname

This behavior matches Firefox and IE11.

TEST=fast/dom/Node/initial-values.html

Review URL: https://codereview.chromium.org/256773004

git-svn-id: svn://svn.chromium.org/blink/trunk@172711 bbb929c8-8fbe-4397-9dbb-9b2b20218538