[css-grid] grid shorthand does not reset gutter properties

The CSS WG has resolved that the "grid" shorthand shouldn't reset
the gutter properties anymore:
https://github.com/w3c/csswg-drafts/issues/1036

This was originally implemented in r370102, so this is somehow
a revert of that change.
The patch adds a new test to WPT verifying the expected behavior.

BUG=761905
TEST=external/wpt/css/css-grid-1/grid-definition/grid-shorthand-001.html

Change-Id: Ib8b0591a0132c6520c3cfd8f13fb2cf2a33e1be0
Reviewed-on: https://chromium-review.googlesource.com/650406
Reviewed-by: Sergio Villar <svillar@igalia.com>
Reviewed-by: Philip Jägenstedt <foolip@chromium.org>
Reviewed-by: meade_UTC10 <meade@chromium.org>
Reviewed-by: Javier Fernandez <jfernandez@igalia.com>
Commit-Queue: Manuel Rego Casasnovas <rego@igalia.com>
Cr-Commit-Position: refs/heads/master@{#499901}
diff --git a/css/css-grid-1/grid-definition/grid-shorthand-001.html b/css/css-grid-1/grid-definition/grid-shorthand-001.html
new file mode 100644
index 0000000..476415e
--- /dev/null
+++ b/css/css-grid-1/grid-definition/grid-shorthand-001.html
@@ -0,0 +1,37 @@
+<!DOCTYPE html>
+<meta charset="utf-8">
+<title>CSS Grid Layout Test: 'grid' shorthand does not reset gutter properties</title>
+<link rel="author" title="Manuel Rego Casasnovas" href="mailto:rego@igalia.com">
+<link rel="help" href="https://drafts.csswg.org/css-grid-1/#grid-shorthand">
+<meta name="flags" content="dom">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<style>
+  #grid {
+    display: grid;
+    grid-column-gap: 10px;
+    grid-row-gap: 20px;
+  }
+</style>
+<div id="grid"></div>
+<script>
+  var grid = document.getElementById("grid");
+
+  test(
+    () => {
+      assert_equals(window.getComputedStyle(grid)["grid-template-columns"], "none");
+      assert_equals(window.getComputedStyle(grid)["grid-template-rows"], "none");
+      assert_equals(window.getComputedStyle(grid)["grid-column-gap"], "10px");
+      assert_equals(window.getComputedStyle(grid)["grid-row-gap"], "20px");
+    }, "Check gutter properties initial values");
+
+  grid.style.grid = "100px / 200px";
+
+  test(
+    () => {
+      assert_equals(window.getComputedStyle(grid)["grid-template-columns"], "200px");
+      assert_equals(window.getComputedStyle(grid)["grid-template-rows"], "100px");
+      assert_equals(window.getComputedStyle(grid)["grid-column-gap"], "10px");
+      assert_equals(window.getComputedStyle(grid)["grid-row-gap"], "20px");
+    }, "Check gutter properties after setting 'grid' shorthand");
+</script>