bindings: Supports [SaveSameObject] extended attribute.

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 394 matching lines @@
 ```js
 window.screenX; // Evaluates to 0
 window.screenX = "foo";
 window.screenX; // Evaluates to "foo"
 delete window.screenX;
 window.screenX; // Evaluates to 0. 0 remains.
 ```
 
 Whether `[Replaceable]` should be specified or not depends on the spec of each attribute.
 
-### [SameObject] _(m)_
+### [SameObject] _(a)_
 
 Standard: [SameObject](http://heycam.github.io/webidl/#SameObject)
 
 Summary: Signals that a `readonly` attribute that returns an object type always returns the same object.
 
-The resulting object is cached in the binding layer and re-used in later calls in order to guarantee that the attribute always returns the same object.
+This attribute has no effect on code generation and should simply be used in Blink IDL files if the specification uses it. If you want the binding layer to cache the resulting object, use `[SaveSameObject]`.
 
 ### [TreatNullAs] _(a,p)_, [TreatUndefinedAs] _(a,p)_
 
 Standard: [TreatNullAs](http://heycam.github.io/webidl/#TreatNullAs)
 
 `[TreatUndefinedAs]` has been been removed from the spec.
 
 Summary: They control the behavior when a JavaScript null or undefined is passed to a DOMString attribute or parameter.
 
 Implementation: **Non-standard:** Web IDL specifies the EmptyString identifier for both these extended attributes, and `Null` and `Missing` for `TreatUndefinedAs`. Blink uses the `NullString` identifier instead of `EmptyString` which yields a Blink null string, corresponding to JavaScript `null`, for which both `String::IsEmpty()` and `String::IsNull()` return true, instead of the empty string `""`, which is empty but not `null`. This is for performance reasons; see [Strings in Blink](https://docs.google.com/a/google.com/document/d/1kOCUlJdh2WJMJGDf-WoEQhmnjKLaOYRbiHz5TiGJl14/edit) for reference. It also does not implement `TreatUndefinedAs=Null` or `TreatUndefinedAs=Missing`.
@@ skipping 808 matching lines @@
 do:
 
 ```webidl
 // Overload can be replaced with optional if `[RuntimeEnabled]` is removed
 foo(long x);
 [RuntimeEnabled=FeatureName] foo(long x, long y);
 ```
 
 For more information, see [RuntimeEnabledFeatures](https://code.google.com/p/chromium/codesearch#chromium/src/third_party/WebKit/Source/platform/RuntimeEnabledFeatures.in).
 
+### [SaveSameObject] _(a)_
+
+Summary: Caches the resulting object and always returns the same object.
+
+When specified, caches the resulting object and returns it in later calls so that the attribute always returns the same object. Must be accompanied with `[SameObject]`.
+
 ### [SetWrapperReferenceFrom=xxx] _(i)_
 
 ### [SetWrapperReferenceTo=xxx] _(i)_
 
 Summary: This generates code that allows you to set up implicit references between wrappers which can be used to keep wrappers alive during GC.
 
 Usage: `[SetWrapperReferenceFrom]` and `[SetWrapperReferenceTo]` can be specified on an interface. Use `[Custom=VisitDOMWrapper]` instead if want to write a custom function.
 
 ```webidl
 [
@@ skipping 295 matching lines @@
 Copyright (C) 2009 Apple Inc. All rights reserved.
 
 Redistribution and use in source and binary forms, with or without modification, are permitted provided that the following conditions are met:
 
 1. Redistributions of source code must retain the above copyright notice, this list of conditions and the following disclaimer.
 
 2. Redistributions in binary form must reproduce the above copyright notice, this list of conditions and the following disclaimer in the documentation and/or other materials provided with the distribution.
 
 THIS SOFTWARE IS PROVIDED BY APPLE INC. AND ITS CONTRIBUTORS “AS IS” AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL APPLE INC. OR ITS CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 ***