Oilpan: Mention pre-finalizer registration in the documentation.

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

 # Blink GC API reference
 
 This document is work in progress.
 
 [TOC]
 
 ## Header file
 
 Unless otherwise noted, any of the primitives explained in this page requires the following `#include` statement:
 
@@ skipping 80 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, preFinalizerName);`".
+
 ```c++
 class YourClass : public GarbageCollectedFinalized<YourClass> {
     USING_PRE_FINALIZER(YourClass, dispose);
 public:
+    YourClass()
+    {
+        ThreadState::current()->registerPreFinalizer(this, dispose);

[sof, 2015/11/21 16:11:36]

ThreadState::registerPreFinalizer() doesn't take two arguments (any longer.)

[Yuta Kitamura, 2015/11/24 07:07:59]

Whoops, thanks. I'll follow-up with this.

(Comments in ThreadState.h also have two arguments version of this
 function, which I'll fix up together.)

+    }
     void dispose()
     {
         m_other->dispose(); // OK; you can touch other on-heap objects in a pre-finalizer.
     }
     ~YourClass()
     {
         // m_other->dispose(); // BAD.
     }
 
 private:
@@ skipping 189 matching lines @@
 DEFINE_TRACE(C)
 {
     visitor->trace(m_x);
     visitor->trace(m_y); // Weak member needs to be traced.
     visitor->trace(m_z); // Heap collection does, too.
     B::trace(visitor); // Delegate to the parent. In this case it's empty, but this is required.
 }
 ```
 
 ## Heap collections