Oilpan: Prepare to move DatabaseContext to Oilpan heap.

Source/modules/webdatabase/DatabaseManager.h

 /*
  * Copyright (C) 2012 Apple Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 81 matching lines @@
 
     PassRefPtrWillBeRawPtr<DatabaseBackendBase> openDatabaseBackend(ExecutionContext*,
         DatabaseType, const String& name, const String& expectedVersion, const String& displayName,
         unsigned long estimatedSize, bool setVersionInNewDatabase, DatabaseError&, String& errorMessage);
 
     static void logErrorMessage(ExecutionContext*, const String& message);
 
     AbstractDatabaseServer* m_server;
 
     // Access to the following fields require locking m_contextMapLock:
+#if ENABLE(OILPAN)
+    // We can't use PersistentHeapHashMap because multiple threads update the map.

[haraken, 2014/03/28 01:03:24]

What's the issue of updating a PersistentHeapHashMap from multiple threads? I
guess it's safe as long as it's guaranteed that a thread that touches the
PersistentHeapHashMap does not outlive the thread that created the
PersistentHeapHashMap.

[tkent, 2014/03/28 01:17:25]

When the (Persistent)HeapHashMap gets new value and it needs to allocate more
memory for backing, it uses heap for the *current thread*.  Even if the value
was removed from the HeapHashMap and the corresponding thread is dying, the
allocated backing can be still used.  Then the thread heap can't be empty.

+    typedef HashMap<ExecutionContext*, OwnPtr<Persistent<DatabaseContext> > > ContextMap;
+#else
     typedef HashMap<ExecutionContext*, RefPtr<DatabaseContext> > ContextMap;
+#endif
     ContextMap m_contextMap;
 #if !ASSERT_DISABLED
     int m_databaseContextRegisteredCount;
     int m_databaseContextInstanceCount;
 #endif
     Mutex m_contextMapLock;
 };
 
 } // namespace WebCore
 
 #endif // DatabaseManager_h