[CSS Grid Layout] Stretch value for align and justify properties.

If the ‘width’ or ‘height’ (as appropriate) of the alignment subject
is ‘auto’, its used value is the length necessary to make the alignment
subject’s outer size as close to the size of the alignment container as
possible, while still respecting the constraints imposed by
‘min/max-width/height’. Otherwise, this is equivalent to ‘start’.

BUG=249451, 376823

Review URL: https://codereview.chromium.org/613273002

git-svn-id: svn://svn.chromium.org/blink/trunk@184782 bbb929c8-8fbe-4397-9dbb-9b2b20218538