[GridNG] Stand up virtual test suite using LayoutNGGrid flag

1. Creating virtual test suite to run all the `css-grid` web tests from
   `fast/css-grid-layout` and `external/wpt/css/css-grid` using the new
   LayoutNG implementation. Test suite is individually disabled in order
   to progressively re-enable tests fixed by new changes.

Bug: 1045599
Change-Id: Ia28d40e8ab75c3668910e66330658f52ffcf2595
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2227306
Commit-Queue: Ian Kilpatrick <ikilpatrick@chromium.org>
Reviewed-by: Kurt Catti-Schmidt <kschmi@microsoft.com>
Reviewed-by: Ian Kilpatrick <ikilpatrick@chromium.org>
Reviewed-by: Christian Biesinger <cbiesinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#775811}

third_party/blink/web_tests/VirtualTestSuites

@@ -253,6 +253,12 @@
               "fast/forms/fieldset"],
     "args": ["--enable-blink-features=LayoutNGFieldset,LayoutNGBlockFragmentation"]
   },
+  {
+    "prefix": "layout-ng-grid",
+    "bases": ["fast/css-grid-layout",
+              "external/wpt/css/css-grid"],
+    "args": ["--enable-blink-features=LayoutNGGrid"]
+  },
   {
     "prefix": "dark-mode-grayscale-images",
     "bases": [],

third_party/blink/web_tests/virtual/layout-ng-grid/external/wpt/css/css-grid/README.txt

+These tests are run with --enable-blink-features=LayoutNGGrid
+The LayoutNG project is described here: http://goo.gl/1hwhfX

third_party/blink/web_tests/virtual/layout-ng-grid/fast/css-grid-layout/README.txt

+These tests are run with --enable-blink-features=LayoutNGGrid
+The LayoutNG project is described here: http://goo.gl/1hwhfX