Add WebFileSystemCallbacks::shouldBlockUntilCompletion()

To propagate that sync operation type to Chrome side.

This is a prerequisite change for worker-code cleanup in
FileSystem API: http://crbug.com/257349

BUG=257349
TEST=no behavioral changes

Review URL: https://chromiumcodereview.appspot.com/18671005

git-svn-id: svn://svn.chromium.org/blink/trunk@154090 bbb929c8-8fbe-4397-9dbb-9b2b20218538