Use visitDOMWrapper to preserve WebGL JS object wrappers

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 841 matching lines @@
 [
     Custom=VisitDOMWrapper,
 ] interface XXX {
     ...
 };
 ```
 
 And then in V8XXXCustom.cpp:
 
 ```c++
-void V8XXX::visitDOMWrapper(DOMDataStore* store, void* object, v8::Persistent<v8::Object> wrapper)
+void V8XXX::visitDOMWrapper(v8::Isolate* isolate, ScriptWrappable* scriptWrappable, v8::Persistent<v8::Object> wrapper)
 {
     ...
 }
+```
+
+
+#### [Custom=VisitDOMWrapperExtra] _(i)_

[haraken, 2016/08/31 23:14:50]

I do want to avoid introducing [Custom=VisitDOMWrapperExtra]. Would you
elaborate on why [Custom=VisitDOMWrapper] is not enough?

[Ken Russell (switch to Gerrit), 2016/09/01 00:08:49]

As mentioned in the summary below, these classes already use the extended IDL
attribute SetWrapperReferenceFrom, which generates a visitDOMWrapper. We don't
want to copy the autogenerated code into a custom visitDOMWrapper, in case the
bindings generator changes.

[Kai Ninomiya, 2016/09/02 18:29:22]

Done.

+
+Summary: Allows you to write extra custom code for visitDOMWrapper, without conflicting with the visitDOMWrapper implementation generated by `[SetWrapperReferenceFrom]`. That implementation will call your visitDOMWrapperExtra implementation. One use (WebGLRenderingContext.idl).
+
+Usage:
+
+```webidl
+[
+    Custom=VisitDOMWrapperExtra,
+] interface XXX {
+    ...
+};
+```
+
+And then in V8XXXCustom.cpp:
+
+```c++
+void V8XXX::visitDOMWrapperExtra(v8::Isolate* isolate, ScriptWrappable* scriptWrappable, v8::Persistent<v8::Object> wrapper)
+{
+    ...
+}
 ```
 
 ### [CustomElementCallbacks] _(m, a)_
 
 Summary: Wraps the method/accessor with a Custom Elements "callback delivery scope" which will dispatch Custom Element callbacks (createdCallback, attributeChangedCallback, etc.) before returning to script.
 
 *** note
 This attribute is only for Custom Elements V0,
 and is superceded by `[CEReactions]` for V1.
 ***
@@ skipping 709 matching lines @@
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 ***