LongTaskTiming: add idl, Experiment flag, PerformanceEntry and basic implementation

third_party/WebKit/Source/core/core.gypi

 {
     'includes': [
         'core_generated.gypi',
     ],
     'variables': {
         # Files for which bindings (.cpp and .h files) will be generated
         'core_idl_files': [
             'animation/KeyframeEffect.idl',
             'animation/AnimationEffectReadOnly.idl',
             'animation/AnimationEffectTiming.idl',
@@ skipping 357 matching lines @@
             'svg/SVGTransform.idl',
             'svg/SVGTransformList.idl',
             'svg/SVGUnitTypes.idl',
             'svg/SVGUseElement.idl',
             'svg/SVGViewElement.idl',
             'svg/SVGViewSpec.idl',
             'timing/MemoryInfo.idl',
             'timing/Performance.idl',
             'timing/PerformanceCompositeTiming.idl',
             'timing/PerformanceEntry.idl',
+            'timing/PerformanceLongTaskTiming.idl',
             'timing/PerformanceMark.idl',
             'timing/PerformanceMeasure.idl',
             'timing/PerformanceNavigation.idl',
             'timing/PerformanceObserver.idl',
             'timing/PerformanceObserverEntryList.idl',
             'timing/PerformanceRenderTiming.idl',
             'timing/PerformanceResourceTiming.idl',
             'timing/PerformanceTiming.idl',
             'timing/WorkerPerformance.idl',
             'workers/SharedWorker.idl',
@@ skipping 1851 matching lines @@
             'timing/DOMWindowPerformance.h',
             'timing/MemoryInfo.cpp',
             'timing/Performance.cpp',
             'timing/Performance.h',
             'timing/PerformanceBase.cpp',
             'timing/PerformanceBase.h',
             'timing/PerformanceCompositeTiming.cpp',
             'timing/PerformanceCompositeTiming.h',
             'timing/PerformanceEntry.cpp',
             'timing/PerformanceEntry.h',
+            'timing/PerformanceLongTaskTiming.cpp',
+            'timing/PerformanceLongTaskTiming.h',
             'timing/PerformanceMark.h',
             'timing/PerformanceMeasure.h',
             'timing/PerformanceNavigation.cpp',
             'timing/PerformanceNavigation.h',
             'timing/PerformanceObserver.cpp',
             'timing/PerformanceObserver.h',
             'timing/PerformanceObserverCallback.h',
             'timing/PerformanceObserverEntryList.cpp',
             'timing/PerformanceObserverEntryList.h',
             'timing/PerformanceRenderTiming.cpp',
@@ skipping 2011 matching lines @@
             'testing/PrivateScriptTestTest.cpp',
             'timing/MemoryInfoTest.cpp',
             'timing/PerformanceBaseTest.cpp',
             'timing/PerformanceObserverTest.cpp',
             'workers/WorkerThreadTest.cpp',
             'workers/WorkerThreadTestHelper.h',
             'xml/parser/SharedBufferReaderTest.cpp',
         ],
     }
 }