Move focus management API from HTMLDocument to Document

Move focus management API from HTMLDocument to Document as per the HTML5
specification:
http://www.whatwg.org/specs/web-apps/current-work/#focus-management

The hasFocus() / activeElement API is now available to both HTML and XML
documents, not only HTML documents.

This change is consistent with the behavior of Firefox 26 and IE 11.

R=tkent, arv
BUG=335788
TEST=fast/dom/Document/xml-document-focus.xml

Review URL: https://codereview.chromium.org/141733006

git-svn-id: svn://svn.chromium.org/blink/trunk@165515 bbb929c8-8fbe-4397-9dbb-9b2b20218538