Implicit prefinalizer registration.

third_party/WebKit/Source/platform/heap/BlinkGCAPIReference.md

 # Blink GC API reference
 
 This is a through document for Oilpan API usage.
 If you want to learn the API usage quickly, look at
 [this tutorial](https://docs.google.com/presentation/d/1XPu03ymz8W295mCftEC9KshH9Icxfq81YwIJQzQrvxo/edit#slide=id.p).
 If you're just interested in wrapper tracing,
 see [Wrapper Tracing Reference](../../bindings/core/v8/TraceWrapperReference.md).
 
 [TOC]
 
@@ skipping 160 matching lines @@
 `USING_PRE_FINALIZER(ClassName, functionName)` in a class declaration declares the class has a *pre-finalizer* of name
 `functionName`.
 
 A pre-finalizer is a user-defined member function of a garbage-collected class that is called when the object is going
 to be swept but before the garbage collector actually sweeps any objects. Therefore, it is allowed for a pre-finalizer
 to touch any other on-heap objects, while a destructor is not. It is useful for doing some cleanups that cannot be done
 with a destructor.
 
 A pre-finalizer must have the following function signature: `void preFinalizer()`. You can change the function name.
 
-A pre-finalizer must be registered in the constructor by using the following statement:
-"`ThreadState::current()->registerPreFinalizer(this);`".
-
 ```c++
 class YourClass : public GarbageCollectedFinalized<YourClass> {
     USING_PRE_FINALIZER(YourClass, dispose);
 public:
-    YourClass()
-    {
-        ThreadState::current()->registerPreFinalizer(this);
-    }
     void dispose()
     {
         m_other->dispose(); // OK; you can touch other on-heap objects in a pre-finalizer.
     }
     ~YourClass()
     {
         // m_other->dispose(); // BAD.
     }
 
 private:
@@ skipping 259 matching lines @@
 };
 ```
 
 Please be very cautious if you want to use a heap collection from multiple threads. Reference to heap collections may be passed to another thread using CrossThreadPersistents, but *you may not modify the collection from the non-owner thread*. This is because modifications to collections may trigger backing store reallocations, and Oilpan's per thread heap requires that modifications to a heap happen on its owner thread.
 
 ### Weak collections
 
 You can put `WeakMember<T>` in heap collections except for `HeapVector` and `HeapDeque` which we do not support.
 
 During an Oilpan GC, the weak members that refernce a collected object will be removed from its heap collection, meaning the size of the collection will shrink and you do not have to check for null weak members when iterating through the collection.