[mathml] Implement MathML token elements (<mtext>, <mi>, <mo>, etc)

These elements are generally text containers for operators, numbers,
variables, comments etc. However, they are also integration points for
foreign markup (SVG and HTML).

See
https://mathml-refresh.github.io/mathml-core/#token-elements
https://mathml-refresh.github.io/mathml-core/#html-and-svg

Bug: 6606
Change-Id: I595222fec2e1d9d582ee3c01f76f4771506e8994
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2391150
Commit-Queue: Rob Buis <rbuis@igalia.com>
Reviewed-by: Morten Stenshorne <mstensho@chromium.org>
Reviewed-by: Frédéric Wang <fwang@igalia.com>
Cr-Commit-Position: refs/heads/master@{#804754}

third_party/blink/renderer/core/css/mathml.css

@@ -47,7 +47,7 @@ math[display="block"] {
     outline: auto 1px -webkit-focus-ring-color;
 }
 
-maction, merror, mfrac, mmultiscripts, mo, mover, mpadded, mphantom, mprescripts, mroot, mrow, msqrt, mspace, mstyle, msub, msup, msubsup, munder, munderover, none
+maction, merror, mfrac, mi, mmultiscripts, mn, mo, mover, mpadded, mphantom, mprescripts, mroot, mrow, ms, msqrt, mspace, mstyle, msub, msup, msubsup, mtext, munder, munderover, none
 {
     display: math;
 }

third_party/blink/renderer/core/layout/layout_object_factory.cc

@@ -47,7 +47,7 @@
 #include "third_party/blink/renderer/core/layout/ng/table/layout_ng_table_column.h"
 #include "third_party/blink/renderer/core/layout/ng/table/layout_ng_table_row.h"
 #include "third_party/blink/renderer/core/layout/ng/table/layout_ng_table_section.h"
-#include "third_party/blink/renderer/core/mathml/mathml_operator_element.h"
+#include "third_party/blink/renderer/core/mathml/mathml_element.h"
 #include "third_party/blink/renderer/core/style/computed_style.h"
 #include "third_party/blink/renderer/platform/runtime_enabled_features.h"
 
@@ -140,7 +140,7 @@ LayoutBlock* LayoutObjectFactory::CreateMath(Node& node,
                                              const ComputedStyle& style,
                                              LegacyLayout legacy) {
   bool disable_ng_for_type = !RuntimeEnabledFeatures::MathMLCoreEnabled();
-  if (IsA<MathMLOperatorElement>(node)) {
+  if (IsA<MathMLElement>(node) && To<MathMLElement>(node).IsTokenElement()) {
     return CreateObject<LayoutBlockFlow, LayoutNGMathMLBlockFlow,
                         LayoutBlockFlow>(node, style, legacy,
                                          disable_ng_for_type);

third_party/blink/renderer/core/layout/ng/ng_block_node.cc

@@ -131,7 +131,8 @@ NOINLINE void DetermineMathMLAlgorithmAndRun(
     } else if (IsA<MathMLPaddedElement>(element)) {
       CreateAlgorithmAndRun<NGMathPaddedLayoutAlgorithm>(params, callback);
       return;
-    } else if (IsA<MathMLOperatorElement>(element)) {
+    } else if (IsA<MathMLElement>(element) &&
+               To<MathMLElement>(*element).IsTokenElement()) {
       CreateAlgorithmAndRun<NGBlockLayoutAlgorithm>(params, callback);
       return;
     } else if (IsA<MathMLScriptsElement>(element) &&

third_party/blink/renderer/core/mathml/mathml_element.cc

@@ -137,4 +137,11 @@ base::Optional<Length> MathMLElement::AddMathLengthToComputedStyle(
   return parsed_value->ConvertToLength(conversion_data);
 }
 
+bool MathMLElement::IsTokenElement() const {
+  return HasTagName(mathml_names::kMiTag) || HasTagName(mathml_names::kMoTag) ||
+         HasTagName(mathml_names::kMnTag) ||
+         HasTagName(mathml_names::kMtextTag) ||
+         HasTagName(mathml_names::kMsTag);
+}
+
 }  // namespace blink

third_party/blink/renderer/core/mathml/mathml_element.h

@@ -33,6 +33,8 @@ class CORE_EXPORT MathMLElement : public Element {
   bool IsMathMLElement() const =
       delete;  // This will catch anyone doing an unnecessary check.
 
+  bool IsTokenElement() const;
+
  protected:
   bool IsPresentationAttribute(const QualifiedName&) const override;
   void CollectStyleForPresentationAttribute(