Add once field to devtools so it can be queried by lighthouse.

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

Index: third_party/WebKit/Source/core/inspector/ThreadDebugger.cpp
diff --git a/third_party/WebKit/Source/core/inspector/ThreadDebugger.cpp b/third_party/WebKit/Source/core/inspector/ThreadDebugger.cpp
index f3e1edfd727892f4c577ff8b78de903e94773b91..8bcebd03c830d744079e8b7d6f3b3937b58a3c34 100644
--- a/third_party/WebKit/Source/core/inspector/ThreadDebugger.cpp
+++ b/third_party/WebKit/Source/core/inspector/ThreadDebugger.cpp
@@ -414,6 +414,8 @@ void ThreadDebugger::getEventListenersCallback(
                        v8::Boolean::New(isolate, info.useCapture));
     createDataProperty(context, listenerObject, v8String(isolate, "passive"),
                        v8::Boolean::New(isolate, info.passive));
+    createDataProperty(context, listenerObject, v8String(isolate, "once"),
+                       v8::Boolean::New(isolate, info.once));
     createDataProperty(context, listenerObject, v8String(isolate, "type"),
                        v8String(isolate, currentEventType));
     v8::Local<v8::Function> removeFunction;