DOMImplementation::document(): remove redundant get().

Follow up r173611 and remove a redundant use of get() in Document accessor.

R=ager@chromium.org
BUG=340522

Review URL: https://codereview.chromium.org/278573003

git-svn-id: svn://svn.chromium.org/blink/trunk@173640 bbb929c8-8fbe-4397-9dbb-9b2b20218538