The CSS WG resolved [1] that Content Alignment should account to the
track sizing algorithm.

The sizing algorithm has been modified so that two new steps (1.5
and 2.5) were added to compute the Content Alignment offsets after
resolving the columns' and rows' sizes respectively.

This change decouples the Content Alignment logic from the tracks
position, so that we can use it as part of the track sizing algorithm.

I also had to store the whole ContentAlignmentData structure in two
class attributes. We need both, position and distribution offsets, to
be used in different parts of the layout logic.

Finally, since the ContentAlignmentData is now used as persistent
memory, we had to remove the STACK_ALLOCATED flag; hence, I think
using DISALLOW_COPY_AND_ASSIGN and let the functions update the
structure's internal field is a more efficient approach to commit
this change.

[1] https://github.com/w3c/csswg-drafts/issues/2557

Added WPT
  * grid-content-distribution-must-account-for-track-sizing-001.html
  * grid-content-distribution-must-account-for-track-sizing-002.html

Bug: 844744
Change-Id: I90019a96b148d3713467e35fde9482d0d39b1ced
Reviewed-on: https://chromium-review.googlesource.com/1067918
Commit-Queue: Javier Fernandez <jfernandez@igalia.com>
Reviewed-by: Manuel Rego Casasnovas <rego@igalia.com>
Cr-Commit-Position: refs/heads/master@{#566412}