Introduce infrastructure for tracing ScriptWrappables.

third_party/WebKit/Source/bindings/core/v8/V8BindingDesign.md

 # Design of V8 bindings
 
 This document explains key concepts in the V8 binding architecture
 except the lifetime management of DOM wrappers.
 See [V8GCController.md](V8GCController.md) to learn the lifetime management.
 
 [TOC]
 
 ## Isolate
 
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 If a debugger is active, the debugger context may be inserted to
 the context stack.
 
 ## World
 
 A world is a concept to sandbox DOM wrappers among content scripts of
 Chrome extensions. There are three kinds of worlds: a main world,
 an isolated world and a worker world.
 A main world is a world where a normal JavaScript downloaded from the web
 is executed.
-An isolated world is a world where a content script of a Chrome extension.
+An isolated world is a world where a content script of a Chrome extension is
+executed.
 An isolate of the main thread has 1 main world and N isolated worlds.
 An isolate of a worker thread has 1 worker world and 0 isolated world.
 [This diagram](https://drive.google.com/file/d/0B1obCOyvTnPKQmJEWkVtOEN2TmM/view?usp=sharing)
 will be helpful to understand the relationship.
 
 All worlds in one isolate share underlying C++ DOM objects,
 but each world has its own DOM wrappers. That way the worlds in one isolate
 can operate on the same C++ DOM object without sharing any DOM wrapper
 among the worlds.
 
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 // iframe.html
 <script>
 </script>
 ```
 
 To make sure that a DOM wrapper is created in a correct context, you need to
 make sure that the current context must be set to the correct context
 whenever you call toV8(). If you're not sure what context to use,
 ask haraken@chromium.org.