Make customElements.define faster

third_party/WebKit/Source/core/dom/custom/README.md

 # Custom Elements
 
 Custom elements let authors create their own elements, with their own
 methods, behavior, and attribute handling. Custom elements shipped in
 M33. We colloquially refer to that version as "v0."
 
 Contact Dominic Cooney
 ([dominicc@chromium.org](mailto:dominicc@chromium.org)) with
 questions.
 
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 ###### V8HTMLElement Constructor
 
 The `HTMLElement` interface constructor. When a custom element is
 created from JavaScript all we have to go on is the constructor (in
 `new.target`); this uses ScriptCustomElementDefinition's state to find
 the definition to use.
 
 ### Memory Management
 
-Once defined, custom element constructors and prototypes have to be
-kept around indefinitely because they could be created in future by
-the parser. On the other hand, we must not leak when a window can no
-longer run script.
+CustomElementRegistry
+implements
+[wrapper tracing](../../../bindings/core/v8/TraceWrapperReference.md)
 
-We use a V8HiddenValue on the CustomElementRegistry wrapper which
-points to a map that keeps constructors and prototypes alive. See
-ScriptCustomElementDefinition.
+CustomElementDefinition, ScriptCustomElementDefinition hold onto the
+callbacks, etc. directly. They become TraceWrapperBase.
+
+Each V8PerContextData has a V8PrivateProperty identifying the custom
+element. The private property stores an int index into the
+CustomElementsRegistry list of definitions.
 
 ## Style Guide
 
 In comments and prose, write custom elements, not Custom Elements, to
 match the HTML standard.
 
 Prefix type names with CustomElement (singular).
 
 ## Testing
 
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    may remove it even sooner, if we have evidence that sites are using
    feature detection correctly.
 
 ## References
 
 These have links to the parts of the DOM and HTML specs which define
 custom elements:
 
 * [WHATWG DOM Wiki: Custom Elements](https://github.com/whatwg/dom/wiki#custom-elements)
 * [WHATWG HTML Wiki: Custom Elements](https://github.com/whatwg/html/wiki#custom-elements)