Bindings: Require [CachedAttribute] method to be const

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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 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]`.
 
 ## Rare Blink-specific IDL Extended Attributes
 
 These extended attributes are rarely used, generally only in one or two places. These are often replacements for `[Custom]` bindings, and may be candidates for deprecation and removal.
 
 ### [CachedAttribute] _(a)_
 
-Summary: For performance optimization, `[CachedAttribute]` indicates that a wrapped object should be cached on a DOM object. Rarely used (only by IndexDB).
+Summary: For performance optimization, `[CachedAttribute]` indicates that a wrapped object should be cached on a DOM object. Rarely used.
 
-Usage: `[CachedAttribute]` can be specified on attributes, and takes a required value, generally called is*Dirty (esp. isValueDirty):
+Usage: `[CachedAttribute]` can be specified on attributes, and takes a required value, generally called is*Dirty (e.g. isValueDirty):
 
 ```webidl
 interface HTMLFoo {
     [CachedAttribute=isKeyDirty] attribute DOMString key;
     [CachedAttribute=isValueDirty] attribute SerializedScriptValue serializedValue;
 };
 ```
 
 Without `[CachedAttribute]`, the key getter works in the following way:
 
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 2. Otherwise, `HTMLFoo::serializedValue()` is called in Blink.
 3. The result of `HTMLFoo::serializedValue()` is deserialized.
 4. The deserialized result is passed to `toJS()` or `ToV8()`, and is converted to a wrapped object.
 5. The wrapped object is cached.
 6. The wrapped object is returned.
 
 *** note
 You should cache attributes if and only if it is really important for performance. Not only does caching increase the DOM object size, but also it increases the overhead of "cache-miss"ed getters. In addition, setters always need to invalidate the cache.
 ***
 
-`[CachedAttribute]` takes a required parameter which the name of a method to call on the implementation object. The method should take void and return bool. Before the cached attribute is used, the method will be called. If the method returns true the cached value is not used, which will result in the accessor being called again. This allows the implementation to both gain the performance benefit of caching (when the conversion to a script value can be done lazily) while allowing the value to be updated. The typical use pattern is:
+`[CachedAttribute]` takes a required parameter which the name of a method to call on the implementation object. The method should be const, take void and return bool. Before the cached attribute is used, the method will be called. If the method returns true the cached value is not used, which will result in the accessor being called again. This allows the implementation to both gain the performance benefit of caching (when the conversion to a script value can be done lazily) while allowing the value to be updated. The typical use pattern is:
 
 ```c++
 // Called internally to update value
 void Object::setValue(Type data)
 {
     m_data = data;
     m_attributeDirty = true;
 }
 
 // Called by generated binding code
-bool Object::isAttributeDirty()
+bool Object::isAttributeDirty() const
 {
     return m_attributeDirty;
 }
 
 // Called by generated binding code if no value cached or isAttributeDirty() returns true
 ScriptValue Object::attribute(ExecutionContext* context)
 {
     m_attributeDirty = false;
     return convertDataToScriptValue(m_data);
 }
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 ***
 
 [CrossOriginProperties]: https://html.spec.whatwg.org/multipage/browsers.html#crossoriginproperties-(-o-)