Special-case LocalDOMWindow for same-origin access in bindings.

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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 used for the `Location.href` attribute: cross-origin writes to this attribute
 are allowed, since it navigates the browsing context, but allowing cross-origin
 reads would leak cross-origin information.
 
 ```webidl
 [CrossOrigin=(Getter,Setter)] readonly attribute Location location;
 ```
 With both `Getter` and `Setter`, allows both cross-origin reads and cross-origin
 writes. This is used for the `Window.location` attribute.
 
+### [SameOriginImplementedAs] _(i)
+
+Summary: Used in conjunction with (#_CrossOrigin_i). Allows a different C++
+class to be used for a same-origin access.
+
+Usage for interfaces:
+```webidl
+[
+    SameOriginImplementedAs=LocalDOMWindow,
+] interface Window {
+    ...
+};
+```
+
 ### [CustomConstructor] _(i)_
 
 Summary: They allow you to write custom bindings for constructors.
 
 Usage: They can be specified on interfaces. _Strongly discouraged._ As with `[Custom]`, it is generally better to modify the code generator. Incompatible with `[Constructor]` – you cannot mix custom constructors and generated constructors.
 
 ```webidl
 [
     CustomConstructor(float x, float y, optional DOMString str),
 ] interface XXX {
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 ***
 
 [CrossOriginProperties]: https://html.spec.whatwg.org/multipage/browsers.html#crossoriginproperties-(-o-)