[CSS Grid Layout] RenderGrid::growGrid refactoring

Now it's called RenderGrid::ensureGridSize and it allows to grow both 
directions at once.
Remove checkings before calling it as they are not needed anymore.

Review URL: https://codereview.chromium.org/319593002

git-svn-id: svn://svn.chromium.org/blink/trunk@175885 bbb929c8-8fbe-4397-9dbb-9b2b20218538