Add another uma metric for HitTesting validity.

Source/core/layout/HitTestCache.cpp

Index: Source/core/layout/HitTestCache.cpp
diff --git a/Source/core/layout/HitTestCache.cpp b/Source/core/layout/HitTestCache.cpp
index 7a37eb7147d716f7fb370782969ec6958a68d81f..8ab3377278f84a396d7067cc8e9b4174c35650ab 100644
--- a/Source/core/layout/HitTestCache.cpp
+++ b/Source/core/layout/HitTestCache.cpp
@@ -38,6 +38,7 @@ void HitTestCache::verifyCachedResult(const HitTestResult& expected, const HitTe
     ValidityHistogramMetric metric;
     if (!actual.equalForCacheability(expected)) {
         metric = expected.hitTestLocation().isRectBasedTest() ? ValidityHistogramMetric::INCORRECT_RECT_BASED_EXACT_MATCH : ValidityHistogramMetric::INCORRECT_POINT_EXACT_MATCH;
+        Platform::current()->histogramSparse("Event.HitTestValidityScore", static_cast<int>(actual.equalityScore(expected)));
 
         // ASSERT that the cache hit is the same as the actual result.
         ASSERT_NOT_REACHED();