Break FormDataBuilder dependency on Document

FormDataBuilder is part of core/platform/network and should not depend on
Document.

R=jochen@chromium.org
BUG=297477

Review URL: https://codereview.chromium.org/25986002

git-svn-id: svn://svn.chromium.org/blink/trunk@158942 bbb929c8-8fbe-4397-9dbb-9b2b20218538