Devirtualize isStretchingChildren

There is no need for this to be virtual if we know the RenderObject is a RenderDeprecatedFlexibleBox, which we do here.
Also clean the method up a bit.

Review URL: https://codereview.chromium.org/154343002

git-svn-id: svn://svn.chromium.org/blink/trunk@167017 bbb929c8-8fbe-4397-9dbb-9b2b20218538