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 be5579a972231b7e31f3d3b92a7c3ccb1bea55fe..fcf0f030641b2b4201c8d09c197076e2a3f42fc0 100644
--- a/third_party/WebKit/Source/core/css/SelectorChecker.cpp
+++ b/third_party/WebKit/Source/core/css/SelectorChecker.cpp
@@ -449,6 +449,8 @@ 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