Use optional default value syntax in Selection.idl

Since now the support for optional default value syntax is present, so
using it now in Selection.idl.

BUG=None

Review URL: https://codereview.chromium.org/519663002

git-svn-id: svn://svn.chromium.org/blink/trunk@181147 bbb929c8-8fbe-4397-9dbb-9b2b20218538