[CSS Grid Layout] Cache track breadth min/max content types

Profiling shows we repeatedly hit the GridLength isLength() method and
the is{Min,Max}Content() methods of the actual Length object. By
caching this information, which won't change until we set again the
grid lengths, we can improve the auto-grid-lots-of-data.html test by
about a 6% (39 runs/s -> 41.5 runs/s).

BUG=234211

Review URL: https://codereview.chromium.org/28053003

git-svn-id: svn://svn.chromium.org/blink/trunk@160184 bbb929c8-8fbe-4397-9dbb-9b2b20218538