Introduce infrastructure for tracing ScriptWrappables.

third_party/WebKit/Source/bindings/IDLExtendedAttributes.md

 # Blink IDL Extended Attributes
 
 [TOC]
 
 ## Introduction
 
 The main interest in extended attributes are their _semantics_: Blink implements many more extended attributes than the Web IDL standard, to specify various behavior.
 
 The authoritative list of allowed extended attributes and values is [bindings/IDLExtendedAttributes.txt](https://code.google.com/p/chromium/codesearch#chromium/src/third_party/WebKit/Source/bindings/IDLExtendedAttributes.txt). This is complete but not necessarily precise (there may be unused extended attributes or values), since validation is run on build, but coverage isn't checked.
 
@@ skipping 1251 matching lines @@
 ```webidl
 [
   SetWrapperReferenceTo=targetMethod
 ] interface YYY { ... };
 ```
 
 The code generates a function called `YYY::visitDOMWrapper` which is called by `V8GCController` before GC. The function adds implicit references from this object's wrapper to a target object's wrapper to keeps it alive.
 
 The `[SetWrapperReferenceTo]` extended attribute takes a value, which is the method name to call to get the target object. For example, with the above declaration a call will be made to `YYY::targetMethod()` to get the target of the reference.
 
+### [TraceWrapperReferences=(list)] _(i)_
+
+Summary: This generates code that traces script-wrappable references (which are used to keep wrappers alive during V8 GC).
+
+Usage: `[TraceWrapperReferences=(list)]` can be specified on an interface.
+
+```webidl
+[
+  TraceWrapperReferences=(element1, element2)
+] interface XXX : YYY { ... };
+```
+
+The generator generates a functions called `XXX::traceWrappers` which is called by `ScriptWrappableHeapTracer` during V8 GC. The function adds references from the XXX instance to this object's element1() and element2() children.

[haraken, 2016/04/18 04:35:41]

functions => function

[Marcel Hlopko, 2016/04/18 11:45:34]

Done.

+
+The generated code is then:
+
+```c++
+DEFINE_TRACE_WRAPPERS(XXX)
+{
+    visitor->traceWrappers(element1());
+    visitor->traceWrappers(element2());
+
+    YYY::traceWrappers(visitor);
+}
+```
+
 ## Rare Blink-specific IDL Extended Attributes
 
 These extended attributes are rarely used, generally only in one or two places. These are often replacements for `[Custom]` bindings, and may be candidates for deprecation and removal.
 
 ### [CachedAttribute] _(a)_
 
 Summary: For performance optimization, `[CachedAttribute]` indicates that a wrapped object should be cached on a DOM object. Rarely used (only by IndexDB).
 
 Usage: `[CachedAttribute]` can be specified on attributes, and takes a required value, generally called is*Dirty (esp. isValueDirty):
 
@@ skipping 267 matching lines @@
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 ***