ActiveScriptWrappable: GC wrappers in detached ExecutionContexts.

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 500 matching lines @@
 ] interface Foo {};
 
 interface Bar : Foo {};  // inherits [ActiveScriptWrappable] from Foo
 ```
 
 If a given DOM object needs to be kept alive as long as the DOM object has pending activities, you need to specify `[ActiveScriptWrappable]` and `[DependentLifetime]`. For example, `[ActiveScriptWrappable]` can be used when the DOM object is expecting events to be raised in the future.
 
 If you use `[ActiveScriptWrappable]`, the corresponding Blink class needs to inherit ActiveScriptWrappable and override hasPendingActivity(). For example, in case of XMLHttpRequest, core/xml/XMLHttpRequest.h would look like this:
 
 ```c++
-class XMLHttpRequest : public ActiveScriptWrappable
+class XMLHttpRequest : public ActiveScriptWrappable<XMLHttpRequest>
 {
     // Returns true if the object needs to be kept alive.
     bool hasPendingActivity() const override { return ...; }
 }
 ```
 
 ### [PerWorldBindings] _(m, a)_
 
 Summary: Generates faster bindings code by avoiding check for isMainWorld().
 
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 ***
 
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