This isn't 100% reproducible step for the assertion failure 

1. Start WinCairo Debug MiniBrowser
2. Close the window
3. ASSERTION FAILED

ASSERTION FAILED: !isMainRunLoop()
C:\home\webkit\gc\Source\WebCore\workers/service/server/SWRegistrationDatabase.cpp(220) : WebCore::SWRegistrationDatabase::close
The thread 0x16b0 has exited with code 0 (0x0).
1   00007FF9509326FB WTFCrash
2   00007FF91A5B33AE WTFCrashWithInfo
3   00007FF91EFF53CF WebCore::SWRegistrationDatabase::close
4   00007FF91EFF2824 WebCore::SWRegistrationDatabase::~SWRegistrationDatabase
5   00007FF94A67A259 WebCore::SWRegistrationDatabase::`scalar deleting destructor'
6   00007FF94A679180 std::default_delete<WebCore::SWRegistrationDatabase>::operator()
7   00007FF94A66C333 std::unique_ptr<WebCore::SWRegistrationDatabase,std::default_delete<WebCore::SWRegistrationDatabase> >::~unique_ptr<WebCore::SWRegistrationDatabase,std::default_delete<WebCore::SWRegistrationDatabase> >
8   00007FF94A66C69B WebKit::ServiceWorkerStorageManager::~ServiceWorkerStorageManager
9   00007FF94A67A2B8 WebKit::ServiceWorkerStorageManager::`scalar deleting destructor'
10  00007FF94A6791D0 std::default_delete<WebKit::ServiceWorkerStorageManager>::operator()
11  00007FF94A66C383 std::unique_ptr<WebKit::ServiceWorkerStorageManager,std::default_delete<WebKit::ServiceWorkerStorageManager> >::~unique_ptr<WebKit::ServiceWorkerStorageManager,std::default_delete<WebKit::ServiceWorkerStorageManager> >
12  00007FF94A61521D WebKit::NetworkStorageManager::~NetworkStorageManager
13  00007FF94A67A038 WebKit::NetworkStorageManager::`scalar deleting destructor'
14  00007FF949A3784E `WTF::ThreadSafeWeakPtrControlBlock::strongDeref<IPC::WorkQueueMessageReceiver,0>'::`2'::<lambda_1>::operator()
15  00007FF949A276C1 WTF::ThreadSafeWeakPtrControlBlock::strongDeref<IPC::WorkQueueMessageReceiver,0>
16  00007FF949A3BD97 WTF::ThreadSafeRefCountedAndCanMakeThreadSafeWeakPtr<IPC::WorkQueueMessageReceiver,0>::deref
17  00007FF949A30803 WTF::Ref<WebKit::NetworkStorageManager,WTF::RawPtrTraits<WebKit::NetworkStorageManager> >::~Ref<WebKit::NetworkStorageManager,WTF::RawPtrTraits<WebKit::NetworkStorageManager> >
18  00007FF94A620C28 ``WebKit::NetworkStorageManager::close'::`2'::<lambda_2>::operator()'::`2'::<lambda_1>::~<lambda_1>
19  00007FF94A630A3B WTF::Detail::CallableWrapper<``WebKit::NetworkStorageManager::close'::`2'::<lambda_2>::operator()'::`2'::<lambda_1>,void>::~CallableWrapper<``WebKit::NetworkStorageManager::close'::`2'::<lambda_2>::operator()'::`2'::<lambda_1>,void>
20  00007FF94A630A78 WTF::Detail::CallableWrapper<``WebKit::NetworkStorageManager::close'::`2'::<lambda_2>::operator()'::`2'::<lambda_1>,void>::`scalar deleting destructor'
21  00007FF95093EF1F std::default_delete<WTF::Detail::CallableWrapperBase<void> >::operator()
22  00007FF95093E613 std::unique_ptr<WTF::Detail::CallableWrapperBase<void>,std::default_delete<WTF::Detail::CallableWrapperBase<void> > >::~unique_ptr<WTF::Detail::CallableWrapperBase<void>,std::default_delete<WTF::Detail::CallableWrapperBase<void> > >
23  00007FF95093E257 WTF::Function<void __cdecl(void)>::~Function<void __cdecl(void)>
24  00007FF9509DD0F2 WTF::RunLoop::performWork
25  00007FF950ABED21 WTF::RunLoop::wndProc
26  00007FF950ABEC84 WTF::RunLoop::RunLoopWndProc
27  00007FF9F4E9E858 CallWindowProcW
28  00007FF9F4E9E299 DispatchMessageW
29  00007FF950ABE166 WTF::RunLoop::run
30  00007FF9496BC67B WebKit::AuxiliaryProcessMainBase<WebKit::NetworkProcess,0>::run
31  00007FF9496BBE83 WebKit::AuxiliaryProcessMain<WebKit::NetworkProcessMainCurl>