Shape-inside should default to 'auto'

As part of the split between shape-outside and shape-inside, the default
value of shape-inside should be 'auto' rather than 'outside-shape'.

BUG=322165

Review URL: https://codereview.chromium.org/92583002

git-svn-id: svn://svn.chromium.org/blink/trunk@163017 bbb929c8-8fbe-4397-9dbb-9b2b20218538