Ship object-fit and object-position.

Change runtime flag ObjectFitPosition from experimental to stable.

BUG=236331,236333

Review URL: https://codereview.chromium.org/25761005

git-svn-id: svn://svn.chromium.org/blink/trunk@158982 bbb929c8-8fbe-4397-9dbb-9b2b20218538