WebIDL constants should be non configurable

Currently we make constants configurable. This change makes us
consistent with the WebIDL spec as well as with Firefox and IE.

BUG=300002

Review URL: https://chromiumcodereview.appspot.com/25109003

git-svn-id: svn://svn.chromium.org/blink/trunk@158475 bbb929c8-8fbe-4397-9dbb-9b2b20218538