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

third_party/WebKit/Source/devtools/front_end/sdk/RuntimeModel.js

Index: third_party/WebKit/Source/devtools/front_end/sdk/RuntimeModel.js
diff --git a/third_party/WebKit/Source/devtools/front_end/sdk/RuntimeModel.js b/third_party/WebKit/Source/devtools/front_end/sdk/RuntimeModel.js
index e733e8a3153d6a0422b7df183c9b0f3e5d0a8101..50265c0bd6d9aa2900f7477bd5ec5c62037fbe00 100644
--- a/third_party/WebKit/Source/devtools/front_end/sdk/RuntimeModel.js
+++ b/third_party/WebKit/Source/devtools/front_end/sdk/RuntimeModel.js
@@ -607,6 +607,7 @@ WebInspector.EventListener = class extends WebInspector.SDKObject {
    * @param {string} type
    * @param {boolean} useCapture
    * @param {boolean} passive
+   * @param {boolean} once
    * @param {?WebInspector.RemoteObject} handler
    * @param {?WebInspector.RemoteObject} originalHandler
    * @param {!WebInspector.DebuggerModel.Location} location
@@ -619,6 +620,7 @@ WebInspector.EventListener = class extends WebInspector.SDKObject {
       type,
       useCapture,
       passive,
+      once,
       handler,
       originalHandler,
       location,
@@ -629,6 +631,7 @@ WebInspector.EventListener = class extends WebInspector.SDKObject {
     this._type = type;
     this._useCapture = useCapture;
     this._passive = passive;
+    this._once = once;
     this._handler = handler;
     this._originalHandler = originalHandler || handler;
     this._location = location;
@@ -660,6 +663,13 @@ WebInspector.EventListener = class extends WebInspector.SDKObject {
   }
 
   /**
+   * @return {boolean}
+   */
+  once() {
+    return this._once;
+  }
+
+  /**
    * @return {?WebInspector.RemoteObject}
    */
   handler() {