Remove dead code from Blink API

Also removed unnecessary duplicate method WebElement::document() (which
is also provided by WebNode).

R=abarth

Review URL: https://codereview.chromium.org/37523003

git-svn-id: svn://svn.chromium.org/blink/trunk@160390 bbb929c8-8fbe-4397-9dbb-9b2b20218538