There was a crash in debug if you use calc()
mixing fixed and percentage values due to the wrong DCHECK
in GapLength constructor. The patch fixes this assert.

In addition LayoutGrid::GridGap() was also wrong and didn't consider
calc() either. The fix is again easy just using the proper check.

Regarding testing, the parsing tests have been updated to include
this combination of fixed and percentage values in calc().
At the same time, the patch actually uses "grid-" prefixed properties
in the tests that were supposed to test those.
Last, two new tests are added to verify the proper behavior of calc()
with mixed values on a grid layout container.

BUG=816300
TEST=external/wpt/css/css-align/gaps/column-gap-parsing-001.html
TEST=external/wpt/css/css-align/gaps/gap-parsing-001.html
TEST=external/wpt/css/css-align/gaps/grid-column-gap-parsing-001.html
TEST=external/wpt/css/css-align/gaps/grid-gap-parsing-001.html
TEST=external/wpt/css/css-align/gaps/grid-row-gap-parsing-001.html
TEST=external/wpt/css/css-align/gaps/row-gap-parsing-001.html
TEST=external/wpt/css/css-grid/alignment/grid-gutters-011.html
TEST=external/wpt/css/css-grid/alignment/grid-gutters-012.html

Change-Id: I4c9fe6b2525a253c6bb00cbda727c2bf1ae6e90b
Reviewed-on: https://chromium-review.googlesource.com/962148
Commit-Queue: Manuel Rego Casasnovas <rego@igalia.com>
Reviewed-by: Sergio Villar <svillar@igalia.com>
Cr-Commit-Position: refs/heads/master@{#543079}