Convert RenderFlexibleBox code to use RenderBox references

RenderFlexibleBox has several methods that take pointers to RenderBox instances but
those methods assumes that the objects will never be NULL. We should replace them by
references. This makes the caller code safer.

Review URL: https://codereview.chromium.org/555213002

git-svn-id: svn://svn.chromium.org/blink/trunk@181787 bbb929c8-8fbe-4397-9dbb-9b2b20218538