Oilpan: Prepare to move DatabaseContext to Oilpan heap.

Source/modules/webdatabase/DatabaseManager.h

Index: Source/modules/webdatabase/DatabaseManager.h
diff --git a/Source/modules/webdatabase/DatabaseManager.h b/Source/modules/webdatabase/DatabaseManager.h
index 861d21987fa5d1568f17213cc59916828fd843ad..f4ee1b794f9596445b1293cbc5560bf33faccd35 100644
--- a/Source/modules/webdatabase/DatabaseManager.h
+++ b/Source/modules/webdatabase/DatabaseManager.h
@@ -99,7 +99,12 @@ private:
     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;