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.
 
@@ skipping 1555 matching lines @@
     v8::FunctionCallback callback)
 {
   v8::experimental::FastAccessorBuilder* fab =
           v8::experimental::FastAccessorBuilder::New(isolate);
   fab->ReturnValue(fab->IntegerContant(123));
   return fab;
 }
 
 ```
 
+
+### [CachedAccessor] _(a)_
+
+Summary: Caches accessor result to a private property.
+

[haraken, 2016/09/20 14:15:02]

Add a webidl example.

+

[haraken, 2016/09/20 14:15:02]

Some description is needed to describe what [CachedAccessor] is. You can move
some of the description from "note".

+*** note
+The accessor cannot have any side effects since calls to the getter will be suppresed, being replaced by a cheap property load.
+It uses a **push approach**, so updates must be 'pushed' every single time, it **WONT'T** invalidate/recalculate the values automatically.
+The original getter can still be used/called on certain circumstances.
+Useful for performance-critical use cases, where the accessor's result doesn't change often.
+
+Please add the required property to V8PrivateProperty.h.
+To update the cache (e.g. for window.document) proceed as follows:
+
+```c++
+V8PrivateProperty::getWindowDocument().set(context, object, newValue);
+```
+
+
+***
+
+Usage: `[CachedAccessor]` takes no arguments, can be specified on attributes.
+
+
 ## 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:
 
@@ skipping 63 matching lines @@
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 ***