[CSS Grid Layout] Rename gridAutoFlow[Row|Column] to gridAutoFlow[Row|Column]Sparse

Rename CSS styles in grid layout tests to make the name more explicit. As the
default mode for auto-placement algorithm is "sparse".

BUG=384099

Review URL: https://codereview.chromium.org/401473003

git-svn-id: svn://svn.chromium.org/blink/trunk@178500 bbb929c8-8fbe-4397-9dbb-9b2b20218538