[CSS Grid Layout] Handle percentages of indefinite sizes in minmax()

After r164062 percentages of indefinite sizes were correctly
computed to "auto". The problem is that we were not doing it when
the percentage was inside the minmax() function. In those cases
it should compute to min-content for the min track sizing
function or to max-content for the max track sizing function.

Added a couple of tests, one to verify that we do not assert with
percentages inside minmax() and another one to verify that we
properly compute the percentage of indefinite size in various
situations.

BUG=401477,409085

Review URL: https://codereview.chromium.org/450093004

git-svn-id: svn://svn.chromium.org/blink/trunk@181800 bbb929c8-8fbe-4397-9dbb-9b2b20218538