DevTools: fix a crash in stopRuleUsageTracking

third_party/WebKit/Source/core/inspector/InspectorCSSAgent.cpp

Index: third_party/WebKit/Source/core/inspector/InspectorCSSAgent.cpp
diff --git a/third_party/WebKit/Source/core/inspector/InspectorCSSAgent.cpp b/third_party/WebKit/Source/core/inspector/InspectorCSSAgent.cpp
index de63a15e1b0eb8c7802aa4b3575546b0fb4c5417..00bcca6123ac82b211e3336b6e16277902eb6b2c 100644
--- a/third_party/WebKit/Source/core/inspector/InspectorCSSAgent.cpp
+++ b/third_party/WebKit/Source/core/inspector/InspectorCSSAgent.cpp
@@ -2527,8 +2527,12 @@ Response InspectorCSSAgent::stopRuleUsageTracking(
 
         StyleRule* styleRule = cssRule->styleRule();
 
-        result->get()->addItem(
-            buildObjectForRuleUsage(cssRule, m_tracker->contains(styleRule)));
+        std::unique_ptr<protocol::CSS::RuleUsage> protocolRule =
+            buildObjectForRuleUsage(cssRule, m_tracker->contains(styleRule));
+        if (!protocolRule)
+          continue;
+
+        result->get()->addItem(std::move(protocolRule));
       }
     }
   }