[CSS Grid] Do not grow grid tracks when the growth factor has 0 length

RenderGrid::distributeSpaceToTracks is by far the hottest method in
the auto-grid-lots-of-data.html performance test. By reducing useless
"growing" of grid tracks when the growth factor is zero we can improve
the test time by 3.55% (422 runs/s -> 437 runs/s).

BUG=234211

Review URL: https://codereview.chromium.org/48573002

git-svn-id: svn://svn.chromium.org/blink/trunk@161354 bbb929c8-8fbe-4397-9dbb-9b2b20218538