IDLExtendedAttributes: Remove outdated note from [Unforgeable] documentation

Stop referencing http://crbug.com/497616; the bug was fixed years ago and
since e56d75ce ("bindings: Supports inheritance of [Unforgeable]
attributes as accessor-type properties") it does not look like unforgeable
attributes are turned into data properties.

Bug: 497616
Change-Id: I79835434ad1a7955f3a0a76ae0a0e07e3d102555
Reviewed-on: https://chromium-review.googlesource.com/980333Reviewed-by: Yuki Shiino <yukishiino@chromium.org>
Reviewed-by: Hitoshi Yoshida <peria@chromium.org>
Commit-Queue: Raphael Kubo da Costa <raphael.kubo.da.costa@intel.com>
Cr-Commit-Position: refs/heads/master@{#545770}

third_party/WebKit/Source/bindings/IDLExtendedAttributes.md

@@ -493,9 +493,7 @@ Usage: Can be specified on methods, attributes or interfaces:
 
 By default, interface members are configurable (i.e. you can modify a property descriptor corresponding to the member and also you can delete the property). `[Unforgeable]` makes the member unconfiguable so that you cannot modify or delete the property corresponding to the member.
 
-`[Unconfiguable]` changes where the member is defined, too. By default, attribute getters/setters and methods are defined on a prototype chain. `[Unforgeable]` defines the member on the instance object instead of the prototype object.
-
-Implementation: **Non-standard**: `[Unforgeable]` for attributes has an unspeced side-effect that it makes the property data-type property (`{writable: ..., value: ...}`) although it must be accessor-type property (`{get: ..., set: ...}`). ([Bug 497616](https://crbug.co/497616))
+`[Unforgeable]` changes where the member is defined, too. By default, attribute getters/setters and methods are defined on a prototype chain. `[Unforgeable]` defines the member on the instance object instead of the prototype object.
 
 ### [Unscopable] _(o, a)_