Make sure resource client is called asynchronously

Source/core/fetch/Resource.cpp

Index: Source/core/fetch/Resource.cpp
diff --git a/Source/core/fetch/Resource.cpp b/Source/core/fetch/Resource.cpp
index 936193b9f27e34bf19c43464216663076a2e5162..31d2d856e8afedeb617cf4350ac9f01458b828e1 100644
--- a/Source/core/fetch/Resource.cpp
+++ b/Source/core/fetch/Resource.cpp
@@ -563,12 +563,18 @@ void Resource::didAccessDecodedData()
 
 void Resource::finishPendingClients()
 {
-    while (!m_clientsAwaitingCallback.isEmpty()) {
-        ResourceClient* client = m_clientsAwaitingCallback.begin()->key;
-        m_clientsAwaitingCallback.remove(client);
-        m_clients.add(client);
-        didAddClient(client);
+    // Clients can be added during the loop. We must not process them now.
+    // So save the current set to the stack.
+    HashCountedSet<ResourceClient*> clientsAwaitingCallback;
+    clientsAwaitingCallback.swap(m_clientsAwaitingCallback);
+
+    for (HashCountedSet<ResourceClient*>::iterator it = clientsAwaitingCallback.begin(); it != clientsAwaitingCallback.end(); ++it) {
+        m_clients.add(it->key);
+        didAddClient(it->key);
     }
+    // It is a critical problem if a callback is scheduled but there is no client waiting for it.
+    // Such a callback cannot be cancelled. It is better to crash the renderer now.
+    RELEASE_ASSERT(!ResourceCallback::callbackHandler()->isScheduled(this) || !m_clientsAwaitingCallback.isEmpty());
 }
 
 void Resource::prune()