Add a WebDocument::watchCssSelectors(selectors)

Source/core/page/RuntimeCSSEnabled.cpp

Index: Source/core/page/RuntimeCSSEnabled.cpp
diff --git a/Source/core/page/RuntimeCSSEnabled.cpp b/Source/core/page/RuntimeCSSEnabled.cpp
index 42934b3ad44776ec7107654d5d61ec3f2c10373b..4db424574b4b965cec20aa239d6260874ba899c9 100644
--- a/Source/core/page/RuntimeCSSEnabled.cpp
+++ b/Source/core/page/RuntimeCSSEnabled.cpp
@@ -118,6 +118,9 @@ static void setPropertySwitchesFromRuntimeFeatures()
     RuntimeCSSEnabled::setCSSPropertyEnabled(CSSPropertyPaintOrder, RuntimeEnabledFeatures::svgPaintOrderEnabled());
     RuntimeCSSEnabled::setCSSPropertyEnabled(CSSPropertyVariable, RuntimeEnabledFeatures::cssVariablesEnabled());
     RuntimeCSSEnabled::setCSSPropertyEnabled(CSSPropertyMaskSourceType, RuntimeEnabledFeatures::cssMaskSourceTypeEnabled());
+
+    // InternalCallback is an implementation detail, rather than an experimental feature, and should never be exposed to the web.
+    RuntimeCSSEnabled::setCSSPropertyEnabled(CSSPropertyInternalCallback, false);
 }
 
 static BoolVector& propertySwitches()