[mathml] Treat unknown MathML elements as <mrow> elements.

The MathML CG decided that unknown elements in the MathML namespace with
a math display should behave like an <mrow> element [1] [2]. This CL
ensures that a MathMLRowElement is created for unknown MathML elements
in order to make that possible. It does not seem necessary to create a
specific MathMLUnknownElement C++ class at that point. Whether a
specific MathMLUnknownElement IDL is needed is still open [1][3].

[1] https://github.com/mathml-refresh/mathml/issues/139
[2] https://mathml-refresh.github.io/mathml-core/#new-display-math-value
[3] https://bugs.chromium.org/p/chromium/issues/detail?id=1021837

Bug: 6606
Change-Id: Ia493392ee8bfe9a7073e2ea6f1ce65927ec7fcae
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2007738
Commit-Queue: Frédéric Wang <fwang@igalia.com>
Reviewed-by: Ian Kilpatrick <ikilpatrick@chromium.org>
Reviewed-by: Mason Freed <masonfreed@chromium.org>
Cr-Commit-Position: refs/heads/master@{#737263}

third_party/blink/renderer/core/dom/document.cc

@@ -247,6 +247,7 @@
 #include "third_party/blink/renderer/core/loader/progress_tracker.h"
 #include "third_party/blink/renderer/core/loader/text_resource_decoder_builder.h"
 #include "third_party/blink/renderer/core/mathml/mathml_element.h"
+#include "third_party/blink/renderer/core/mathml/mathml_row_element.h"
 #include "third_party/blink/renderer/core/mathml_element_factory.h"
 #include "third_party/blink/renderer/core/mathml_names.h"
 #include "third_party/blink/renderer/core/origin_trials/origin_trial_context.h"
@@ -1022,10 +1023,11 @@ Element* Document::CreateRawElement(const QualifiedName& qname,
   } else if (RuntimeEnabledFeatures::MathMLCoreEnabled() &&
              qname.NamespaceURI() == mathml_names::kNamespaceURI) {
     element = MathMLElementFactory::Create(qname.LocalName(), *this, flags);
+    // An unknown MathML element is treated like an <mrow> element.
     // TODO(crbug.com/1021837): Determine if we need to introduce a
-    // MathMLUnknownClass.
+    // MathMLUnknownElement IDL.
     if (!element)
-      element = MakeGarbageCollected<MathMLElement>(qname, *this);
+      element = MakeGarbageCollected<MathMLRowElement>(qname, *this);
     saw_elements_in_known_namespaces_ = true;
   } else {
     element = MakeGarbageCollected<Element>(qname, this);

third_party/blink/renderer/core/mathml/mathml_tag_names.json5

@@ -2,7 +2,7 @@
   metadata: {
     namespace: "MathML",
     namespaceURI: "http://www.w3.org/1998/Math/MathML",
-    fallbackInterfaceName: "MathMLElement",
+    fallbackInterfaceName: "MathMLRowElement",
     fallbackJSInterfaceName: "MathMLElement",
     export: "CORE_EXPORT",
   },