Include wpt/css/CSS2/ in LayoutNGFragmentTraversal testing.

The failure / skip expectation lines were copied from non-virtual
TestExpectations, FlagExpectations/layout-ng-fragment-item and
NeverFixTests. No failures were introduced by enabling
LayoutNGFragmentTraversal itself.

Bug: 1043787
Change-Id: I26a8cff3a700abbb076e309d5162c1da3440136b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2036096Reviewed-by: Ian Kilpatrick <ikilpatrick@chromium.org>
Reviewed-by: Koji Ishii <kojii@chromium.org>
Commit-Queue: Koji Ishii <kojii@chromium.org>
Cr-Commit-Position: refs/heads/master@{#738552}

third_party/blink/web_tests/VirtualTestSuites

@@ -199,7 +199,8 @@
   },
   {
     "prefix": "layout_ng_fragment_traversal",
-    "bases": ["fast/block",
+    "bases": ["external/wpt/css/CSS2",
+              "fast/block",
               "fast/table"],
     "args": ["--enable-blink-features=LayoutNGFragmentTraversal"]
   },