DevTools: [lighthouse] Implement performance metrics filmstrip

third_party/WebKit/Source/devtools/front_end/platform/utilities.js

 /*
  * Copyright (C) 2007 Apple Inc.  All rights reserved.
  * Copyright (C) 2012 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  *
  * 1.  Redistributions of source code must retain the above copyright
  *     notice, this list of conditions and the following disclaimer.
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   function windowLoaded() {
     self.removeEventListener('DOMContentLoaded', windowLoaded, false);
     callback();
   }
 
   if (document.readyState === 'complete' || document.readyState === 'interactive')
     callback();
   else
     self.addEventListener('DOMContentLoaded', windowLoaded, false);
 }
+
+var _singletonSymbol = Symbol('singleton');
+
+/**
+ * @template T
+ * @param {function(new:T, ...)} constructorFunction
+ * @return {!T}
+ */
+function singleton(constructorFunction) {
+  if (_singletonSymbol in constructorFunction)
+    return constructorFunction[_singletonSymbol];
+  var instance = new constructorFunction();
+  constructorFunction[_singletonSymbol] = instance;
+  return instance;
+}