Reland [css-grid] Implement auto-repeat computation

Original CL (1895443002) was reverted because it triggered several
use-of-uninitialized-value errors on Linux WebKit Linux MSan bot. See for details.

This CL properly initializes the two variables that were used before being
initialized using the appropriate ComputedStyle::initializeXXX() methods.

Original issue's description:
> [css-grid] Implement auto-repeat computation
> We added support for parsing and style for the new auto-repeat syntax. This
> CL actually implements the feature by computing the number of auto-repeat
> tracks that will be part of the explicit grid depending on the available
> size on the corresponding axis.
> Note that this CL does not drop the empty tracks, i.e., auto-fit will work
> exactly as auto-fill until the empty track removal is implemented.
> Test results are not totally correct yet because we need to add the line
> names to the computed style. As that requires a rather large piece of code
> it will be done in a follow up patch. Track sizes are correct though.


Review URL:

Cr-Commit-Position: refs/heads/master@{#389746}
19 files changed