Remove support for anonymous deprecated flexboxes.

There are no remaining internal clients of RenderDeprecatedFlexibleBox,
so removing support for anonymous deprecated flexboxes.

These changes has been taken from following WebKit patch
http://trac.webkit.org/changeset/155689

Review URL: https://codereview.chromium.org/363343005

git-svn-id: svn://svn.chromium.org/blink/trunk@179655 bbb929c8-8fbe-4397-9dbb-9b2b20218538