DevTools: fix a crash in stopRuleUsageTracking

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

Index: third_party/WebKit/Source/core/inspector/InspectorStyleSheet.cpp
diff --git a/third_party/WebKit/Source/core/inspector/InspectorStyleSheet.cpp b/third_party/WebKit/Source/core/inspector/InspectorStyleSheet.cpp
index 491ab7414c84af364cb6200d647ba57e4337da58..0f73dc24b68fd28ca4c634a63e3a82e6062938df 100644
--- a/third_party/WebKit/Source/core/inspector/InspectorStyleSheet.cpp
+++ b/third_party/WebKit/Source/core/inspector/InspectorStyleSheet.cpp
@@ -1609,6 +1609,9 @@ InspectorStyleSheet::buildObjectForRuleUsage(CSSRule* rule, bool wasUsed) {
 
   CSSRuleSourceData* sourceData = sourceDataForRule(rule);
 
+  if (!sourceData)
+    return nullptr;
+
   std::unique_ptr<protocol::CSS::RuleUsage> result =
       protocol::CSS::RuleUsage::create()
           .setStyleSheetId(id())