Use counter for tabStop.

Source/core/dom/Element.cpp

Index: Source/core/dom/Element.cpp
diff --git a/Source/core/dom/Element.cpp b/Source/core/dom/Element.cpp
index 5dfcd1fc6da28f908bcf73e18c992e52aa4cf061..a1f067d40d2850970a6eac67e81b2f5ca3908a4b 100644
--- a/Source/core/dom/Element.cpp
+++ b/Source/core/dom/Element.cpp
@@ -2100,6 +2100,7 @@ void Element::parseAttribute(const QualifiedName& name, const AtomicString& valu
             setTabIndexExplicitly(max(static_cast<int>(std::numeric_limits<short>::min()), std::min(tabindex, static_cast<int>(std::numeric_limits<short>::max()))));
         }
     } else if (RuntimeEnabledFeatures::tabStopAttributeEnabled() && name == tabstopAttr) {
+        UseCounter::count(document(), UseCounter::TabStopAttribute);
         if (!hasAttribute(tabstopAttr)) {
             // tabstop attribute removed.
             clearElementFlag(TabStopWasSetExplicitly);