createElement('image') should create a HTMLElement

Create a HTMLElement when using createElement('image'), but keep
the HTML5 parser constructing HTMLImageElement for it.

BUG=305773

Review URL: https://codereview.chromium.org/26757002

git-svn-id: svn://svn.chromium.org/blink/trunk@159920 bbb929c8-8fbe-4397-9dbb-9b2b20218538