[mathml] Enable MathML Core tests and add expectations

The status of the MathMLCore runtime flag is changed to "test" so that
that flag is used for running tests. A new section for MathML-related
expectation is introduced in TestExpectations. In addition to
expectations for new tests, the ones of MathML-related CSS tests are
moved there.

Blink currently contains an internal MathML-specific test
(mathml/fallback.html) which verifies fallback via the <semantics>
element when MathML is not implemented. This test will fail when more
MathML patches land but it is still important to ensure that test passes
when MathMLCore is disabled. A new virtual test suite is introduced for
that purpose and the test is moved there.

Expectation for http/tests/xmlviewer/dumpAsText/mathml.xml is also
updated since MathML is recognized as a known namespace when MathMLCore
is enabled.

Bug: 6606
Change-Id: Ia5a657e02850cad142d53a08baf10ec499467726
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1893878
Commit-Queue: Frédéric Wang <fwang@igalia.com>
Reviewed-by: Luke Z <lpz@chromium.org>
Reviewed-by: Manuel Rego <rego@igalia.com>
Cr-Commit-Position: refs/heads/master@{#713440}

third_party/blink/renderer/platform/runtime_enabled_features.json5

@@ -927,6 +927,7 @@
     },
     {
       name: "MathMLCore",
+      status:"test",
     },
     {
       name:"MeasureMemory",

third_party/blink/web_tests/NeverFixTests

@@ -1944,9 +1944,6 @@ virtual/speech-with-unified-autoplay/external/wpt/speech-api/SpeechRecognition-o
 virtual/speech-with-unified-autoplay/external/wpt/speech-api/SpeechRecognition-stop-manual.https.html [ Skip ]
 [ Retina ] external/wpt/pointerevents/pointerevent_touch-action-table-test_touch-manual.html [ Skip ]
 
-# MathML is not implemented.
-crbug.com/6606 external/wpt/mathml [ Skip ]
-
 # ==== Tests incompatible with the default WPT Origin Isolation start here ==VV
 # The section below lists web tests that are incompatible with the WPT Origin
 # Isolation mode of Site Isolation (see

third_party/blink/web_tests/VirtualTestSuites

@@ -619,6 +619,11 @@
     "bases": [],
     "args": ["--enable-font-antialiasing"]
   },
+  {
+    "prefix": "mathml-disabled",
+    "bases": [],
+    "args": ["--disable-blink-features=MathMLCore"]
+  },
   {
     "prefix": "hdr",
     "bases": [],

third_party/blink/web_tests/http/tests/xmlviewer/dumpAsText/mathml-expected.txt

-This XML file does not appear to have any style information associated with it. The document tree is shown below.
-
-<!--
- If you can see this, this test has failed, except if you are
-     not using a MathML-aware client. 
--->
-<FAIL xmlns:a="http://www.w3.org/1998/Math/MathML">
-<header>
-This tests that xml viewer is not used when there is a tag in MATHML namespace.
-</header>
-<a:math>
-<a:mrow>
-<a:mi>SUCCESS</a:mi>
-</a:mrow>
-</a:math>
-</FAIL>
+This tests that xml viewer is not used when there is a tag in MATHML namespace. SUCCESS

third_party/blink/web_tests/virtual/mathml-disabled/README.txt

+This suite runs tests with --disable-blink-features=MathMLCore