[css-grid] Allow percentage values for column and row gutters

The grid-column-gap and grid-row-gap allow now percentage as one of the
possible values. This patch adds the required support for both parsing
and layout logic.

This is based on a patch by Javier Fernández http://crrev.com/2057113002/.
However it uses a different approach to compute the percentage gaps
based on the sizing operation.
During intrinsic size phase the percentage gaps are considered 0px.
Also for indefinite heights the percentage row gaps are 0px too.


Review-Url: https://codereview.chromium.org/2323793002
Cr-Commit-Position: refs/heads/master@{#417386}
6 files changed