Remove Document::getElementById()

Remove Document::getElementById() as it was merely calling TreeScope::getElementById().
Document already inherits from TreeScope, therefore this indirection is not needed.

R=haraken
BUG=

Review URL: https://chromiumcodereview.appspot.com/23995011

git-svn-id: svn://svn.chromium.org/blink/trunk@157763 bbb929c8-8fbe-4397-9dbb-9b2b20218538