[mathml] Implement basic operator support

Set rspace internal style property instead of lspace.

Bug: 6606
Change-Id: If9d3fe5c2bb233dda789868467e11be20e7f25d4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2379683Reviewed-by: Frédéric Wang <fwang@igalia.com>
Reviewed-by: Oriol Brufau <obrufau@igalia.com>
Commit-Queue: Rob Buis <rbuis@igalia.com>
Cr-Commit-Position: refs/heads/master@{#802654}

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

@@ -25,7 +25,7 @@ void MathMLOperatorElement::AddMathRSpaceIfNeeded(
     const CSSToLengthConversionData& conversion_data) {
   if (auto length_or_percentage_value = AddMathLengthToComputedStyle(
           conversion_data, mathml_names::kRspaceAttr)) {
-    style.SetMathLSpace(std::move(*length_or_percentage_value));
+    style.SetMathRSpace(std::move(*length_or_percentage_value));
   }
 }