Add a UseCounter for '>>>' combinator.

third_party/WebKit/Source/core/css/SelectorChecker.cpp

Index: third_party/WebKit/Source/core/css/SelectorChecker.cpp
diff --git a/third_party/WebKit/Source/core/css/SelectorChecker.cpp b/third_party/WebKit/Source/core/css/SelectorChecker.cpp
index 283ffa0a4a28c583d9cdf1aab23bd2553cfabd32..cf7962a053b4e02c078adcdb760aeab78b5574e9 100644
--- a/third_party/WebKit/Source/core/css/SelectorChecker.cpp
+++ b/third_party/WebKit/Source/core/css/SelectorChecker.cpp
@@ -448,6 +448,8 @@ SelectorChecker::Match SelectorChecker::matchForRelation(
 
     case CSSSelector::ShadowPiercingDescendant: {
       DCHECK(m_isQuerySelector);
+      UseCounter::count(context.element->document(),
+                        UseCounter::CSSShadowPiercingDescendantCombinator);
       // TODO(kochi): parentOrOpenShadowHostElement() is necessary because
       // SelectorQuery can pass V0 shadow roots. All closed shadow roots are
       // already filtered out, thus once V0 is removed this logic can use