[CSS Grid Layout] Simplify or better encapsulate the grid position resolution code

Move resolution code to GridSpan::create* methods when possible.

This should simplify RenderGrid a bit, since all that code does not
depend on RenderGrid functionality and can be moved away.

BUG=258258

Review URL: https://codereview.chromium.org/50313003

git-svn-id: svn://svn.chromium.org/blink/trunk@160941 bbb929c8-8fbe-4397-9dbb-9b2b20218538