[css-grid] fit-content() tracks shouldn't stretch

The spec is pretty clear regarding this
(https://drafts.csswg.org/css-grid/#algo-stretch):
  "This step sizes expands tracks that have an auto max track sizing
  function by dividing any remaining positive, definite free space
  equally amongst them."

The fit-content() tracks don't have an auto max track sizing function,
so they shouldn't be considered in this step of the algorithm.

The patch modifies GridTrackSizingAlgorithm::InitializeTrackSizes()
so it doesn't consider fit-content() tracks as auto sized ones
(which are the ones later stretched in the last step of the algorithm).

Added new case to an existent test and also modified
the expected result in another one.
On top of that add a new test in WPT covering more cases.

BUG=755994
TEST=fast/css-grid-layout/grid-content-alignment-stretch-only-valid-for-auto-sized-tracks.html
TEST=external/wpt/css/css-grid-1/alignment/grid-fit-content-tracks-dont-stretch-001.html

Change-Id: Ic4ffb83518f0e0ca879196c5bb4dd997c373c66b
Reviewed-on: https://chromium-review.googlesource.com/618707
Commit-Queue: Manuel Rego Casasnovas <rego@igalia.com>
Reviewed-by: Sergio Villar <svillar@igalia.com>
Reviewed-by: Javier Fernandez <jfernandez@igalia.com>
Cr-Commit-Position: refs/heads/master@{#503437}
6 files changed