[CSSGridLayout] getComputedStyle is wrong for grid-template-{rows|columns}

Specs mandate to return the used value instead of the specified value which
was the one we were using. This requires to force a layout to compute the
used values. The properties grid-auto-{rows|columns} are not affected by
this change as the same specified value might result in different used
values for two different auto-generated tracks.

Tests have been accordingly updated. Fixed size grids are now being used to
produce consistent results. Some new test cases were added to check that we
behave correctly with/without grid sizes and with/without grid items (for
content-based track sizes).

BUG=297158

Review URL: https://codereview.chromium.org/25646010

git-svn-id: svn://svn.chromium.org/blink/trunk@161247 bbb929c8-8fbe-4397-9dbb-9b2b20218538