Add [PrefixGet] 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.
 
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 ### [PostMessage] _(m)_
 
 Summary: Tells the code generator to generate postMessage method used in Workers, Service Workers etc.
 
 Usage: `[PostMessage]` can be specified on methods
 
 ```webidl
 [PostMessage] void postMessage(any message, optional sequence<Transferable> transfer);
 ```
 
+### [PrefixGet] _(d)_
+
+Summary: If this extended attribute is specified on a dictionary member, the code generator adds 'get' prefix to the getter method of the member.
+
 ### [RaisesException] _(i, m, a)_
 
 Summary: Tells the code generator to append an `ExceptionState&` argument when calling the Blink implementation.
 
 Implementations may use the methods on this parameter (e.g. `ExceptionState::throwDOMException`) to throw exceptions.
 
 Usage: `[RaisesException]` can be specified on methods and attributes, `[RaisesException=Getter]` and `[RaisesException=Setter]` can be specified on attributes, and `[RaisesException=Constructor]` can be specified on interfaces where `[Constructor]` is also specified. On methods and attributes, the IDL looks like:
 
 ```webidl
 interface XXX {
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 [CrossOriginProperties]: https://html.spec.whatwg.org/multipage/browsers.html#crossoriginproperties-(-o-)