Add section for heap collections to BlinkGC documentation

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]
 
 ## Header file
 
 Unless otherwise noted, any of the primitives explained in this page requires the following `#include` statement:
 
 ```c++
 #include "platform/heap/Handle.h"
 ```
@@ skipping 401 matching lines @@
 ```
 
 Please notice that if the object (of type `C`) is also not reachable, its `trace` method
 will not be invoked and any follow-on weak processing will not be done. Hence, if the
 object must always perform some operation when the weak reference is cleared, that
 needs to (also) happen during finalization.
 
 Weak callbacks have so far seen little use in Blink, but a mechanism that's available.
 
 ## Heap collections
+
+Heap collections are WTF collection types that support `Member<T>`, `WeakMember<T>`(see [below](#Weak collections)), and garbage collected objects as its elements.
+
+Here is the complete list:
+
+- WTF::Vector → blink::HeapVector
+- WTF::Deque → blink::HeapDeque
+- WTF::HashMap → blink::HeapHashMap
+- WTF::HashSet → blink::HeapHashSet
+- WTF::LinkedHashSet → blink::HeapLinkedHashSet
+- WTF::ListHashSet → blink::HeapListHashSet
+- WTF::HashCountedSet → blink::HeapHashCountedSet
+
+These heap collections work mostly the same way as their WTF collection counterparts but there are some things to keep in mind.
+
+Heap collections are special in that the types themselves do not inherit from GarbageCollected (hence they are not allocated on the Oilpan heap) but they still *need to be traced* from the trace method (because we need to trace the backing store which is on the Oilpan heap).

[haraken, 2016/12/02 12:50:46]

Shall we add some code example?

[keishi, 2016/12/05 01:51:28]

Done.

+
+When you want to add a heap collection as a member of a non-garbage-collected class, please use the persistent variants (just prefix the type with Persistent e.g. PersistentHeapVector, PersistentHeapHashMap, etc.).

[haraken, 2016/12/02 12:50:46]

Ditto.

[keishi, 2016/12/05 01:51:29]

Done.

+
+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.

[haraken, 2016/12/02 12:50:46]

I'm just curious but do we have a runtime verification to detect the
multi-thread access?

[keishi, 2016/12/05 01:51:29]

We don't check for every modification (i.e. the verification in
Member::checkPointer won't work if Member was initialized with memset), but we
do check on reallocation (backingExpand/backingShrink).

+
+### Weak collections
+
+You can use `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.