[CSS Grid Layout] infinity should be defined as a negative value

[CSS Grid Layout] infinity should be defined as a negative value

This change makes us reject negative lengths at parse time, which
matches the updated specification, so that we can now safely assume
that when we resolve the length, it is not to a negative value.

BUG=234209
TESTS=fast/css-grid-layout/grid-columns-rows-get-set.html
      fast/css-grid-layout/grid-columns-rows-get-set-multiple.html

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=154805

[Julien - ping for review, 2013-07-23 20:29:56]

Simple parsing change for grid that cleans up some code!

[ojan, 2013-07-24 01:05:21]

lgtm

[commit-bot: I haz the power, 2013-07-24 01:05:34]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/jchaffraix@chromium.org/19668006/1

[commit-bot: I haz the power, 2013-07-24 03:52:33]

Change committed as 154805