Fix CrossThreadCopier for ThreadableLoaderOptions and ResourceLoaderOptions

When copying ThreadableLoaderOptions and ResourceLoaderOptions by
CrossThreadCopier, make isolatedCopy of AtomicString contained in them and
pass it as String.

BUG=396520

Review URL: https://codereview.chromium.org/409383002

git-svn-id: svn://svn.chromium.org/blink/trunk@178898 bbb929c8-8fbe-4397-9dbb-9b2b20218538