[CSSRegions] Create auto-size folder and move related regions tests

Create fast/regions/auto-size folder and move autosize* and autowidth* tests.
Will move autoheight* in a follow-up patch.

BUG=306369

Review URL: https://codereview.chromium.org/26386005

git-svn-id: svn://svn.chromium.org/blink/trunk@159583 bbb929c8-8fbe-4397-9dbb-9b2b20218538