Add virtual test suite with WCv0-disabled

========================
NOTE TO SHERIFFS: This CL may cause some flaky tests within the
(new) virtual/web-components-v0-disabled suite. If so, please
do not revert this CL; instead mark the flaky tests as SKIP
in the section of TestExpectations marked with crbug.com/937746,
and feel free to notify me or file a bug against me. Thanks!
========================

Because the Web Components v0 features are set to "test", they
still run enabled for layout and WPT tests. This virtual test
suite manually disables them for the DOM-related tests to make
sure our tests are functional without WCv0 features.

Bug: 937746
Change-Id: I3914a2d6614a4cfa0115e486ce5222d4c3539507
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1952088
Commit-Queue: Mason Freed <masonfreed@chromium.org>
Auto-Submit: Mason Freed <masonfreed@chromium.org>
Reviewed-by: Stephen Chenney <schenney@chromium.org>
Cr-Commit-Position: refs/heads/master@{#722611}

third_party/blink/web_tests/VirtualTestSuites

@@ -628,5 +628,23 @@
     "prefix": "not-split-http-cache",
     "bases": ["external/wpt/fetch/http-cache"],
     "args": ["--disable-features=SplitCacheByNetworkIsolationKey"]
+  },
+  {
+    "prefix": "web-components-v0-disabled",
+    "bases": ["external/wpt/dom",
+              "external/wpt/shadow-dom",
+              "external/wpt/html/dom",
+              "external/wpt/html/rendering",
+              "external/wpt/html/semantics",
+              "external/wpt/html/syntax",
+              "dom",
+              "shadow-dom",
+              "html",
+              "fast/dom",
+              "fast/events",
+              "fast/forms",
+              "fast/html",
+              "fast/parser"],
+    "args": ["--disable-blink-features=ShadowDOMV0,CustomElementsV0,HTMLImports"]
   }
 ]

third_party/blink/web_tests/virtual/web-components-v0-disabled/README.md

+This directory contains tests run with the Web Components v0 features disabled.
+
+See http://go/web-components-v0 for more details.
+
+

third_party/blink/web_tests/virtual/web-components-v0-disabled/README.txt

+This directory contains tests run with the Web Components v0 features disabled.
+
+See http://go/web-components-v0 for more details.
+
+