Enable per thread heap for database thread

third_party/WebKit/Source/modules/webdatabase/SQLTransactionBackend.h

 /*
  * Copyright (C) 2007, 2013 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
@@ skipping 87 matching lines @@
 
     SQLTransactionState unreachableState();
     SQLTransactionState sendToFrontendState();
 
     SQLTransactionState nextStateForCurrentStatementError();
     SQLTransactionState nextStateForTransactionError();
     SQLTransactionState runCurrentStatementAndGetNextState();
 
     void getNextStatement();
 
-    Member<SQLTransaction> m_frontend;
-    Member<SQLStatementBackend> m_currentStatementBackend;
+    CrossThreadPersistent<SQLTransaction> m_frontend;

[keishi, 2016/06/08 04:56:16]

This change is for crbug.com/616984.
I had to change these to CrossThreadPersistents.
SQLTransactionBackend is in the main ThreadHeap
And SQLTransaction/SQLStatementBackend is in the main ThreadHeap as well
But these are being set from the database thread.
The database thread should not be modifying the object graph in the main
ThreadHeap.
The main thread could be tracing while the database thread is modifying the
object graph.
So it makes sense to use a CrossThreadPersistent.
TSAN detected this problem as a data race.

I found a same kind of problem in SQLiteDatabase:m_authorizer

I used an assert like this to check locally. I worry this might be too heavy to
land.
https://codereview.chromium.org/2050463003

+    CrossThreadPersistent<SQLStatementBackend> m_currentStatementBackend;
 
     Member<Database> m_database;
     Member<SQLTransactionWrapper> m_wrapper;
     OwnPtr<SQLErrorData> m_transactionError;
 
     bool m_hasCallback;
     bool m_hasSuccessCallback;
     bool m_hasErrorCallback;
     bool m_shouldRetryCurrentStatement;
     bool m_modifiedDatabase;
     bool m_lockAcquired;
     bool m_readOnly;
     bool m_hasVersionMismatch;
 
     Mutex m_statementMutex;
-    HeapDeque<Member<SQLStatementBackend>> m_statementQueue;
+    Deque<CrossThreadPersistent<SQLStatementBackend>> m_statementQueue;
 
     OwnPtr<SQLiteTransaction> m_sqliteTransaction;
 };
 
 } // namespace blink
 
 #endif // SQLTransactionBackend_h