[CSS Shapes] Image valued shape-outside shapes should update the layout after the image has been loaded

Ensure that the an image-valued shape-outside layout is updated after the image has been loaded. Most of the logic is similar to what's done for shape-inside image-values.

BUG=304331

Review URL: https://codereview.chromium.org/26468004

git-svn-id: svn://svn.chromium.org/blink/trunk@159631 bbb929c8-8fbe-4397-9dbb-9b2b20218538