Worker: Introduce an observation mechanism for WorkerThread termination

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/EffectModel.idl',
             'animation/AnimationEffectReadOnly.idl',
@@ skipping 2271 matching lines @@
             'workers/WorkerLoaderProxy.h',
             'workers/WorkerLocation.h',
             'workers/WorkerNavigator.cpp',
             'workers/WorkerNavigator.h',
             'workers/WorkerOrWorkletGlobalScope.h',
             'workers/WorkerReportingProxy.h',
             'workers/WorkerScriptLoader.cpp',
             'workers/WorkerScriptLoader.h',
             'workers/WorkerThread.cpp',
             'workers/WorkerThread.h',
+            'workers/WorkerThreadLifecycleObserver.cpp',
+            'workers/WorkerThreadLifecycleObserver.h',
             'workers/WorkerThreadStartupData.cpp',
             'workers/WorkerThreadStartupData.h',
             'workers/WorkletGlobalScope.cpp',
             'workers/WorkletGlobalScope.h',
             'xml/DOMParser.cpp',
             'xml/DOMParser.h',
             'xml/DocumentXPathEvaluator.cpp',
             'xml/DocumentXPathEvaluator.h',
             'xml/DocumentXSLT.cpp',
             'xml/DocumentXSLT.h',
@@ skipping 1871 matching lines @@
             'testing/PrivateScriptTestTest.cpp',
             'timing/MemoryInfoTest.cpp',
             'timing/PerformanceBaseTest.cpp',
             'timing/PerformanceObserverTest.cpp',
             'workers/WorkerThreadTest.cpp',
             'workers/WorkerThreadTestHelper.h',
             'xml/parser/SharedBufferReaderTest.cpp',
         ],
     }
 }