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

third_party/WebKit/Source/bindings/IDLExtendedAttributes.md

Index: third_party/WebKit/Source/bindings/IDLExtendedAttributes.md
diff --git a/third_party/WebKit/Source/bindings/IDLExtendedAttributes.md b/third_party/WebKit/Source/bindings/IDLExtendedAttributes.md
index 13113cbc5804e70db598dbf67d53845481232e63..d3f88224727f14f69ebf5db6a9cd3e1cae60175c 100644
--- a/third_party/WebKit/Source/bindings/IDLExtendedAttributes.md
+++ b/third_party/WebKit/Source/bindings/IDLExtendedAttributes.md
@@ -1573,6 +1573,20 @@ v8::experimental::FastAccessorBuilder* V8Node::XXXAttributeGetterBuilder(v8::Iso
 
 ```
 
+
+### [StoreInField] _(a)_
+
+Summary: Caches invariable accessors to a private property. Private property name is automatically generated e.g. "Class#attribute".

[haraken, 2016/09/20 05:53:25]

Make it clear that this caches a return value of DOM attribute getters that
don't have any side effect. You shouldn't use this feature for DOM operations
that may have side effects.

[Alfonso, 2016/09/20 13:48:23]

Done.

+
+
+*** note
+Useful for performance critical scenarios, where the accessor does little or none computation at all (peek a field), removing the calling overhead.
+Calls to the getter will be translated into a faster property load.
+***
+
+Usage: `[StoreInField]` 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.