Add [CachedAccessor] attribute to cache (almost) constant accessors (window.document).

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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 These extended attributes are _temporary_ and are only in use while some change is in progress. Unless you are involved with the change, you can generally ignore them, and should not use them.
 
 ### [ExperimentalCallbackFunction]
 
 Summary: `[ExperimentalCallbackFunction]` on a callback function is a flag to collect callback functions. Currently the code generator doesn't generate bindings for IDL callback functions (instead, it just uses `ScriptValue`). While generating bindings for callback functions, to change existing code which uses callback functions until the generated bindings are stabilized is undesirable. To implement bindings generation for IDL callback functions incrementally, [ExperimentalCallbackFunction] extended attribute is added. The code generator keeps using ScriptValue for IDL callback functions which don't have this extended attribute.
 
 ### [LegacyTreatAsPartialInterface] _(i)_
 
 Summary: `[LegacyTreatAsPartialInterface]` on an interface that is the target of an `implements` statement means that the interface is treated as a partial interface, meaning members are accessed via static member functions in a separate class, rather than as instance methods on the instance object `*impl` or class methods on the C++ class implementing the (main) interface. This is legacy from original implementation of `implements`, and is being removed ([Bug 360435](https://crbug.com/360435), nbarth@).
 
+
+### [CachedAccessor] _(a)_
+
+Summary: Caches the accessor result in a private property (not directly accesible from JS). Improves accessor reads (getter) at the expense of extra memory and manual invalidation which should be trivial in most cases.
+
+
+*** note
+* The getter cannot have any side effects since calls to the getter will be replaced by a cheap property load.
+* It uses a **push approach**, so updates must be _pushed_ every single time, it **DOES NOT** invalidate/update the cache automatically.
+* Despite being cached, the getter can still be called on certain circumstances, consistency is a must.
+* The cache **MUST** be initialized before using it. There's no default value like _undefined_ or a _hole_.
+***
+
+
+Usage: `[CachedAccessor]` takes no arguments, can be specified on attributes.
+
+```webidl
+interface HTMLFoo {
+    [CachedAccessor] readonly attribute Bar bar;
+};
+```
+
+
+Register the required property in V8PrivateProperty.h.
+To update the cached value (e.g. for HTMLFoo.bar) proceed as follows:
+
+```c++
+V8PrivateProperty::getHTMLFooBarCachedAccessor().set(context, object, newValue);
+```
+
+
 ## Discouraged Blink-specific IDL Extended Attributes
 
 These extended attributes are _discouraged_ - they are not deprecated, but they should be avoided and removed if possible.
 
 ### [DoNotCheckConstants] _(i)_
 
 Summary: `[DoNotCheckConstants]` indicates that constant values in an IDL file can be different from constant values in Blink implementation.
 
 Usage: `[DoNotCheckConstants]` can be specified on interfaces:
 
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 ***