[mathml] Make MathMLCore depend on LayoutNG

MathML is only supported when LayoutNG is enabled, so make
the dependency between them clear.

Bug: 6606
Change-Id: Ifcd1314c22d333a4e4ec584b231be14241ea304a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1930800Reviewed-by: Ian Kilpatrick <ikilpatrick@chromium.org>
Commit-Queue: Rob Buis <rbuis@igalia.com>
Cr-Commit-Position: refs/heads/master@{#718236}

third_party/blink/renderer/platform/runtime_enabled_features.json5

@@ -925,6 +925,7 @@
     {
       name: "MathMLCore",
       status:"test",
+      depends_on: ["LayoutNG"],
     },
     {
       name:"MeasureMemory",