Implement WeakReference support in Apache Harmony Concurent GC

by simon.zhou for The Apache Software Foundation

Apache Harmony has a concurrent GC(Tick) which performs garbage collection without suspending application threads. Tick don’t support weak reference now, it treats all the references as strong references,which may cause inefficiency for some applications.I will add this feature, it would be different from the implementation in gen GC,since the consistency should be maintained for the objects. Read barrier of get method of reference object will be used,and performance issues will be considered.