During thread detach, delay final sweep until shutdown lock is held.

Source/platform/heap/ThreadState.cpp

Index: Source/platform/heap/ThreadState.cpp
diff --git a/Source/platform/heap/ThreadState.cpp b/Source/platform/heap/ThreadState.cpp
index dcb601d0303c5530c240cb241aecbff9eef83b17..e872e433d319ace02a402ef969be09ca5b30ed79 100644
--- a/Source/platform/heap/ThreadState.cpp
+++ b/Source/platform/heap/ThreadState.cpp
@@ -428,8 +428,6 @@ void ThreadState::cleanupPages()
 void ThreadState::cleanup()
 {
     checkThread();
-    // Finish sweeping.
-    completeSweep();
     {
         // Grab the threadAttachMutex to ensure only one thread can shutdown at
         // a time and that no other thread can do a global GC. It also allows
@@ -439,6 +437,9 @@ void ThreadState::cleanup()
         // GC.
         SafePointAwareMutexLocker locker(threadAttachMutex(), NoHeapPointersOnStack);
 
+        // Finish sweeping.
+        completeSweep();
+
         // From here on ignore all conservatively discovered
         // pointers into the heap owned by this thread.
         m_isTerminating = true;