Combine Element and StyledElement

StyledElement didn't add any data members and almost every Element subclass is
actually a StyledElement. The only ones that aren't are PseudoElement and
WebVTTElement. Having the subclass meant putting lots of casts all over which
is a bit silly since only Elements inside XHR documents, or ones created
through document.implementation.createDocument().createElement() are ever
non-StyledElement.

In the future we should decide if having the distinction even matters, why
isn't every Element just stylable?

TEST=None; no change in behavior.
R=arv@chromium.org, dglazkov@chromium.org

Review URL: https://codereview.chromium.org/17632002

git-svn-id: svn://svn.chromium.org/blink/trunk@152967 bbb929c8-8fbe-4397-9dbb-9b2b20218538