FileAPI: Deprecate openFileSystem and deleteFileSystem in FileSystemClient


BUG=257349
TEST=should pass existing tests (no behavioral changes)

Review URL: https://chromiumcodereview.appspot.com/23600039

git-svn-id: svn://svn.chromium.org/blink/trunk@157696 bbb929c8-8fbe-4397-9dbb-9b2b20218538