[CSS Grid Layout] RenderGrid methods should take RenderBox references

RenderGrid has several methods that take pointers to RenderBox
instances but the body of those methods assumes that the objects
will never be NULL. We should replace them by references.

Review URL: https://codereview.chromium.org/554293003

git-svn-id: svn://svn.chromium.org/blink/trunk@181722 bbb929c8-8fbe-4397-9dbb-9b2b20218538