[v8 gc] Introduce a base class for all objects that can have pending activity

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.
 
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 * `[ImplementedAs]` is only necessary for these legacy files: otherwise the class (C++) implementing (IDL) implemented interfaces does not need to be specified, as this is handled in Blink C++.
 
 * `[OriginTrialEnabled]` behaves as for partial interfaces.
 
 * `[RuntimeEnabled]` behaves as for partial interfaces.
 
 * `[NoInterfaceObject]` is _always_ specified on implemented interfaces.
 
 ### Inheritance
 
-Extended attributes are generally not inherited: only extended attributes on the interface itself are consulted. However, there are a handful of extended attributes that are inherited (applying them to an ancestor interface applies them to the descendants). These are extended attributes that affect memory management, and currently consists of `[DependentLifetime]`; the up-to-date list is [compute_dependencies.INHERITED_EXTENDED_ATTRIBUTES](https://code.google.com/p/chromium/codesearch#chromium/src/third_party/WebKit/Source/bindings/scripts/compute_dependencies.py&q=INHERITED_EXTENDED_ATTRIBUTES).
+Extended attributes are generally not inherited: only extended attributes on the interface itself are consulted. However, there are a handful of extended attributes that are inherited (applying them to an ancestor interface applies them to the descendants). These are extended attributes that affect memory management, and currently consists of `[DependentLifetime]` and `[ActiveScriptWrappable]`; the up-to-date list is [compute_dependencies.INHERITED_EXTENDED_ATTRIBUTES](https://code.google.com/p/chromium/codesearch#chromium/src/third_party/WebKit/Source/bindings/scripts/compute_dependencies.py&q=INHERITED_EXTENDED_ATTRIBUTES).
 
 ## Standard Web IDL Extended Attributes
 
 These are defined in the [ECMAScript-specific extended attributes](http://heycam.github.io/webidl/#es-extended-attributes) section of the [Web IDL spec](http://heycam.github.io/webidl/), and alter the binding behavior.
 
 *** note
 Unsupported: `[ArrayClass]`, `[ImplicitThis]`, `[LenientThis]`, `[NamedPropertiesObject]`, `[TreatNonCallableAsNull]`
 ***
 
 ### [Clamp] _(a, p)_
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 Standard: [Unscopeable](http://heycam.github.io/webidl/#Unscopeable)
 
 Summary: The interface member will not appear as a named property within `with` statements.
 
 Usage: Can be specified on attributes or interfaces.
 
 ## Common Blink-specific IDL Extended Attributes
 
 These extended attributes are widely used.
 
+### [ActiveScriptWrappable] _(i)_
+
+Summary: `[ActiveScriptWrappable]` indicates that a given DOM object should be kept alive as long as the DOM object has pending activities.
+
+Usage: `[ActiveScriptWrappable]` can be specified on interfaces, and **is inherited**:
+
+```webidl
+[
+    ActiveScriptWrappable,
+] interface Foo {
+    ...
+};
+```
+
+If an interface X has `[ActiveScriptWrappable]` and an interface Y inherits the interface X, then the interface Y will also have `[ActiveScriptWrappable]`. For example
+
+```webidl
+[
+    ActiveScriptWrappable,
+] 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. For example, in case of XMLHttpRequest, core/xml/XMLHttpRequest.h would look like this:
+
+```c++
+class XMLHttpRequest : public ActiveScriptWrappable
+{
+    ...;
+}
+```
+
 ### [PerWorldBindings] _(m, a)_
 
 Summary: Generates faster bindings code by avoiding check for isMainWorld().
 
 This optimization only makes sense for wrapper-types (i.e. types that have a corresponding IDL interface), as we don't need to check in which world we are for other types.
 
 *** note
 This optimization works by generating 2 separate code paths for the main world and for isolated worlds. As a consequence, using this extended attribute will increase binary size and we should refrain from overusing it.
 ***
 
@@ skipping 370 matching lines @@
 [
     DependentLifetime,
 ] interface Foo { ... };
 
 interface Bar : Foo { ... };  // inherits [DependentLifetime]
 ```
 
 If a DOM object does not have `[DependentLifetime]`, V8's GC collects the wrapper of the DOM object
 if the wrapper is unreachable on the JS side (i.e., V8's GC assumes that the wrapper should not be
 reachable in the DOM side). Use `[DependentLifetime]` to relax the assumption.
-For example, if the DOM object implements hasPendingActivity(), it must be annotated with
+For example, if the DOM object has `[ActiveScriptWrappable]` and implements hasPendingActivity(), it must be annotated with
 `[DependentLifetime]`. Otherwise, the wrapper will be collected regardless of the returned value
 of the hasPendingActivity(). DOM objects that are pointed to by `[SetWrapperReferenceFrom]` and
 `[SetWrapperReferenceTo]` must be annotated with `[DependentLifetime]`.
 
 ### [DeprecateAs] _(m, a, c)_
 
 Summary: Measures usage of a deprecated feature via UseCounter, and notifies developers about deprecation via a console warning.
 
 `[DeprecateAs]` can be considered an extended form of `[MeasureAs]`: it both measures the feature's usage via the same UseCounter mechanism, and also sends out a warning to the console (optionally with a message) in order to inform developers that the code they've written will stop working at some point in the relatively near future.
 
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 Copyright (C) 2009 Apple Inc. All rights reserved.
 
 Redistribution and use in source and binary forms, with or without modification, are permitted provided that the following conditions are met:
 
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 ***