Filesystem: Replace FileSystemFlags with an IDL dictionary

To remove raw Dictionary use in filesystem.
Tests under http/tests/filesystem cover this change.

BUG=403150

Review URL: https://codereview.chromium.org/616243003

git-svn-id: svn://svn.chromium.org/blink/trunk@183029 bbb929c8-8fbe-4397-9dbb-9b2b20218538