Bug 188283

Regression(r234486): assertion hit in ~CallbackAggregator():
Thread 5 Crashed:: Dispatch queue: com.apple.CFNetwork.CookieServerEndpoint-queue-0x7f9c2500d300
0   com.apple.JavaScriptCore      	0x000000011d1872d0 WTFCrash + 16 (Assertions.cpp:267)
1   com.apple.WebKit              	0x0000000109cfb08b WTFCrashWithInfo(int, char const*, char const*, int) + 27 (Assertions.h:545)
2   com.apple.WebKit              	0x0000000109dd4fcf WTF::CallbackAggregator::~CallbackAggregator() + 127 (CallbackAggregator.h:40)
3   com.apple.WebKit              	0x0000000109dd4f45 WTF::CallbackAggregator::~CallbackAggregator() + 21 (CallbackAggregator.h:43)
4   com.apple.WebKit              	0x0000000109dd4f17 WTF::ThreadSafeRefCounted<WTF::CallbackAggregator, (WTF::DestructionThread)0>::deref() const + 71 (ThreadSafeRefCounted.h:76)
5   com.apple.WebKit              	0x000000010a12fe81 void WTF::derefIfNotNull<WTF::CallbackAggregator>(WTF::CallbackAggregator*) + 49 (RefPtr.h:46)
6   com.apple.WebKit              	0x000000010a12fe49 WTF::RefPtr<WTF::CallbackAggregator, WTF::DumbPtrTraits<WTF::CallbackAggregator> >::~RefPtr() + 41 (RefPtr.h:70)
7   com.apple.WebKit              	0x000000010a12e5a5 WTF::RefPtr<WTF::CallbackAggregator, WTF::DumbPtrTraits<WTF::CallbackAggregator> >::~RefPtr() + 21 (RefPtr.h:70)
8   com.apple.WebKit              	0x000000010a1306a5 auto WebKit::NetworkProcess::platformSyncAllCookies(WTF::CompletionHandler<void ()>&&)::$_2::operator()<WebCore::NetworkStorageSession const>(WebCore::NetworkStorageSession const&) const::'lambda'()::~() + 21 (NetworkProcessCocoa.mm:223)
9   com.apple.WebKit              	0x000000010a130625 auto WebKit::NetworkProcess::platformSyncAllCookies(WTF::CompletionHandler<void ()>&&)::$_2::operator()<WebCore::NetworkStorageSession const>(WebCore::NetworkStorageSession const&) const::'lambda'()::~() + 21 (NetworkProcessCocoa.mm:223)
10  com.apple.WebKit              	0x000000010a130609 __destroy_helper_block_ + 25 (NetworkProcessCocoa.mm:223)
11  libsystem_blocks.dylib        	0x00007fff76e5c988 _Block_release + 109
12  libsystem_blocks.dylib        	0x00007fff76e5c988 _Block_release + 109
13  libsystem_blocks.dylib        	0x00007fff76e5c988 _Block_release + 109
14  libsystem_blocks.dylib        	0x00007fff76e5c988 _Block_release + 109
15  libdispatch.dylib             	0x00007fff76dc3de3 _dispatch_client_callout + 8
16  libdispatch.dylib             	0x00007fff76dca148 _dispatch_lane_serial_drain + 618
17  libdispatch.dylib             	0x00007fff76dcac00 _dispatch_lane_invoke + 388
18  libdispatch.dylib             	0x00007fff76dd30b3 _dispatch_workloop_worker_thread + 603
19  libsystem_pthread.dylib       	0x00007fff770025b4 _pthread_wqthread + 409
20  libsystem_pthread.dylib       	0x00007fff770023ad start_wqthread + 13

On trunk, CallbackAggregators need to be destroyed on the thread they were created on (since r234288). Unfortunately, the CFNetwork _saveCookies SPI is calling us back on a background queue, even though we are calling it on the main thread.