[ABANDONED] Implement ThenableCoercions for AP2 Promises

Implement ThenableCoercions for AP2 Promises

ThenableCoercions is used for AP2 Promises[1] and it is represented as
a WeakMap. However, V8 WeakMap is not exposed to embedders[2] and
it cannot be used if the harmony collections flag is not set.

Since V8 GC relocates the Object addresses, we cannot implement WeakMap
without V8 internal information. So, at this time, we implement
Weak Linked List instead. As a result, the search becomes linear.
When V8 WeakMap becomes available, we replace the implementation.

[1]: https://github.com/domenic/promises-unwrapping
[2]: https://code.google.com/p/v8/issues/detail?id=2908

BUG=295420

[yusukesuzuki, 2013-09-26 02:44:14]

https://codereview.chromium.org/24403002/diff/4001/Source/bindings/v8/Thenabl...
File Source/bindings/v8/ThenableCoercions.cpp (right):

https://codereview.chromium.org/24403002/diff/4001/Source/bindings/v8/Thenabl...
Source/bindings/v8/ThenableCoercions.cpp:75: if (!storedThenable.IsEmpty() &&
storedThenable->SameValue(thenable))
To follow the WeakMap spec of ES6 draft, use SameValue instead of StrictEquals.
SameValue algorithm is exposed by V8 in this CL[1].

[1]: https://codereview.chromium.org/24360017/

[yusukesuzuki, 2013-09-27 01:38:25]

I'm planning to use V8 hidden property instead of ThenableCoercions in the
Promise draft.
It makes the code and the role of WeakMap clear I think.

I'll show the implementation in the AP2 Promises implementation CL. Let's
discuss about Pros/Cons there :)