Move Document pointer from Frame to LocalFrame.

RemoteFrames do not need Document members, so this belongs in the
LocalFrame class and clients who access Document need to ensure that
they are handling a LocalFrame.

R=dcheng@chromium.org, eseidel@chromium.org
BUG=346764

Review URL: https://codereview.chromium.org/235553006

git-svn-id: svn://svn.chromium.org/blink/trunk@171584 bbb929c8-8fbe-4397-9dbb-9b2b20218538