First implementation of lazily cached accessor for DOM attributes.

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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 *** note
 * The getter cannot have any side effects since calls to the getter will be replaced by a cheap property load.
 * It uses a **push approach**, so updates must be _pushed_ every single time, it **DOES NOT** invalidate/update the cache automatically.
 * Despite being cached, the getter can still be called on certain circumstances, consistency is a must.
 * The cache **MUST** be initialized before using it. There's no default value like _undefined_ or a _hole_.
 ***
 
 
-Usage: `[CachedAccessor]` takes no arguments, can be specified on attributes.
+Usage: `[CachedAccessor]` takes an optional argument "Lazy", can be specified on attributes.
 
 ```webidl
 interface HTMLFoo {
     [CachedAccessor] readonly attribute Bar bar;
 };
 ```
 
+```webidl
+interface HTMLFoo {
+    [CachedAccessor=Lazy] readonly attribute Bar bar;
+};
+```
 
 Register the required property in V8PrivateProperty.h.
 To update the cached value (e.g. for HTMLFoo.bar) proceed as follows:
 
 ```c++
 V8PrivateProperty::getHTMLFooBarCachedAccessor().set(context, object, newValue);
 ```
 
 
 ## Discouraged Blink-specific IDL Extended Attributes
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 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.
 ***
 
 [CrossOriginProperties]: https://html.spec.whatwg.org/multipage/browsers.html#crossoriginproperties-(-o-)