[CSS Grid Layout] Percentages of indefinite sizes should compute to "auto"

According to the spec, http://dev.w3.org/csswg/css-grid/#valuedef-percentage,
that's what we have to do. We were not considering this case as something
special in the current code.

Added several new test cases including some that verify that tracks are sized
as per their children (following "auto" behaviour).

BUG=313215

Review URL: https://codereview.chromium.org/60633008

git-svn-id: svn://svn.chromium.org/blink/trunk@164062 bbb929c8-8fbe-4397-9dbb-9b2b20218538