Explicit notification and removal of lifecycle observers.

Source/core/workers/WorkerThread.cpp

Index: Source/core/workers/WorkerThread.cpp
diff --git a/Source/core/workers/WorkerThread.cpp b/Source/core/workers/WorkerThread.cpp
index c60511544ab3c579e55ce404dcde5ef77705a1b8..9a4286b12bfd6006e7346105ca4927f3c26cc772 100644
--- a/Source/core/workers/WorkerThread.cpp
+++ b/Source/core/workers/WorkerThread.cpp
@@ -135,23 +135,31 @@ void WorkerThread::workerThread()
 
     ThreadIdentifier threadID = m_threadID;
 
-    // The below assignment will destroy the context, which will in turn notify messaging proxy.
-    // We cannot let any objects survive past thread exit, because no other thread will run GC or otherwise destroy them.
-    // If Oilpan is enabled, we detach of the context/global scope, with the final heap cleanup below sweeping it out.
-#if ENABLE(OILPAN)
-    m_workerGlobalScope->dispose();
-#else
+#if !ENABLE(OILPAN)
     ASSERT(m_workerGlobalScope->hasOneRef());
 #endif
+    // Dispose of the WorkerGlobalScope and its context. If Oilpan is not enabled, clearing
+    // m_workerGlobalScope synchronously destructs the object. If Oilpan is enabled,
+    // that finalization will instead happen when a final GC is run below during
+    // ThreadState::detach().
+    m_workerGlobalScope->dispose();
     m_workerGlobalScope = nullptr;
 
-    ThreadState::detach();
+    // Notify the proxy that the WorkerGlobalScope has been disposed of.
+    // This can free this thread object, hence it must not be touched afterwards.
+    workerReportingProxy().workerGlobalScopeDestroyed();
 
     // Clean up PlatformThreadData before WTF::WTFThreadData goes away!
     PlatformThreadData::current().destroy();
 
     // The thread object may be already destroyed from notification now, don't try to access "this".
     detachThread(threadID);
+
+    // And, finally, detach the ThreadState, cleaning out the thread's
+    // heap by performing a final GC. The cleanup operation will at
+    // the end assert that the heap is empty, as any surviving objects
+    // will otherwise leak and never be destructed.
+    ThreadState::detach();
 }
 
 void WorkerThread::runEventLoop()