Almost break FormData's dependency on core

This CL removes the dependency on Document and moves FormDataList into
core/platform/network. We still have a dependency on File and Blob, which we'll
need to work out.

R=jochen@chromium.org
BUG=297477

Review URL: https://codereview.chromium.org/25702007

git-svn-id: svn://svn.chromium.org/blink/trunk@158954 bbb929c8-8fbe-4397-9dbb-9b2b20218538