Add [CEReactions] IDL attributes for Custom Elements V1

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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 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]`
 ***
 
+### [CEReactions] _(m, a)_
+
+Standard: [CEReactions](https://html.spec.whatwg.org/multipage/scripting.html#cereactions)
+
+Summary: `[CEReactoins]` indicates that
+[custom element reactions](https://html.spec.whatwg.org/multipage/scripting.html#concept-custom-element-reaction)
+are triggered for this method or attribute.
+
+Usage: `[CEReactions]` takes no arguments.
+
 ### [Clamp] _(a, p)_
 
 Standard: [Clamp](https://heycam.github.io/webidl/#Clamp)
 
 Summary: `[Clamp]` indicates that when an ECMAScript Number is converted to the IDL type, out of range values will be clamped to the range of valid values, rather than using the operators that use a modulo operation (ToInt32, ToUint32, etc.).
 
 Usage: The `[Clamp]` extended attribute MUST NOT appear on a read only attribute, or an attribute, operation argument or dictionary member that is not of an integer type.
 
 `[Clamp]` can be specified on writable attributes:
 
@@ skipping 694 matching lines @@
 void V8XXX::visitDOMWrapper(DOMDataStore* store, void* object, 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.
+***
+
 If the method/accessor creates elements or modifies DOM nodes in any way, it should be tagged with this extended attribute. Even if you're not a Node, this may apply to you! For example [DOMTokenList.toggle](https://code.google.com/p/chromium/codesearch#chromium/src/third_party/WebKit/Source/core/dom/DOMTokenList.idl&l=34) can be reflected in the attribute of its associated element, so it needs to be tagged with CustomElementCallbacks. If the method/accessor only calls something that may modify the DOM (for example, it runs user script as a callback) you don't need to tag your method with `[CustomElementCallbacks]`; that is the responsibility of the binding that actually modifies the DOM. In general over-applying this extended attribute is safe, with one caveat:
 
 * This extended attribute MUST NOT be used on members that operate on non-main threads, because the callback delivery scope accesses statics.
 * Basically: Don't apply this extended attribute to anything that can be called from a worker.
 * This criterion (accessible by workers) depends on implementation and cannot easily be checked from the IDL or C++ headers (it includes obvious cases like `[Exposed=Worker]`, where there is a constructor on the (JS) global object, but also cases where the C++ creates or accesses methods internally), so please be careful.
 
 Usage: `[CustomElementCallbacks]` takes no arguments.
 
 ### [Default] _(p)_
 
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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:
 
 1. Redistributions of source code must retain the above copyright notice, this list of conditions and the following disclaimer.
 
 2. Redistributions in binary form must reproduce the above copyright notice, this list of conditions and the following disclaimer in the documentation and/or other materials provided with the distribution.
 
 THIS SOFTWARE IS PROVIDED BY APPLE INC. AND ITS CONTRIBUTORS “AS IS” AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL APPLE INC. OR ITS CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 ***