Make createEvent do case-insensitive matches for certain interface values as specified here:
http://dom.spec.whatwg.org/#dom-document-createevent

Update the python generator scripts for this and add tests to create-event.html.

This behavior matches Firefox and IE11.

Review URL: https://codereview.chromium.org/250933005

git-svn-id: svn://svn.chromium.org/blink/trunk@173331 bbb929c8-8fbe-4397-9dbb-9b2b20218538