Creating custom element should not be case sensitive

This patch makes the creation of custom elements through type
extension, to be case insenstive.

BUG=282467

Review URL: https://codereview.chromium.org/442343002

git-svn-id: svn://svn.chromium.org/blink/trunk@180255 bbb929c8-8fbe-4397-9dbb-9b2b20218538