[SharedArrayBuffer] Add "AllowShared" extended attribute, used for WebGL

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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 ```
 
 ### [FlexibleArrayBufferView]
 
 Summary: `[FlexibleArrayBufferView]` wraps a parameter that is known to be an ArrayBufferView (or a subtype of, e.g. typed arrays) with a FlexibleArrayBufferView.
 
 The FlexibleArrayBufferView itself can then either refer to an actual ArrayBufferView or a temporary copy (for small payloads) that may even live on the stack. The idea is that copying the payload on the stack and referring to the temporary copy saves creating global handles (resulting in weak roots) in V8. Note that `[FlexibleArrayBufferView]`  will actually result in a TypedFlexibleArrayBufferView wrapper for typed arrays.
 
 Usage: Applies to arguments of methods. See modules/webgl/WebGLRenderingContextBase.idl for an example.
 
+### [AllowShared] _(p)_
+
+Summary: `[AllowShared]` indicates that a parameter, which must be an ArrayBufferView (or subtype of, e.g. typed arrays), is allowed to be backed by a SharedArrayBuffer.
+
+Usage: `[AllowShared]` must be specified on a parameter to a method:
+
+```webidl
+interface Context {
+    void bufferData1([AllowShared] ArrayBufferView buffer);
+    void bufferData2([AllowShared] Float32Array buffer);
+}
+```
+
+A SharedArrayBuffer is a distinct type from an ArrayBuffer, but both types use ArrayBufferViews to view the data in the buffer. Most methods do not permit an ArrayBufferView that is backed by a SharedArrayBuffer, and will throw an exception. This attribute indicates that this method permits a shared ArrayBufferView.
+
 ### [PermissiveDictionaryConversion] _(p, d)_
 
 Summary: `[PermissiveDictionaryConversion]` relaxes the rules about what types of values may be passed for an argument of dictionary type.
 
 Ordinarily when passing in a value for a dictionary argument, the value must be either undefined, null, or an object. In other words, passing a boolean value like true or false must raise TypeError. The PermissiveDictionaryConversion extended attribute ignores non-object types, treating them the same as undefined and null. In order to effect this change, this extended attribute must be specified both on the dictionary type as well as the arguments of methods where it is passed. It exists only to eliminate certain custom bindings.
 
 Usage: applies to dictionaries and arguments of methods. Takes no arguments itself.
 
 ### [URL] _(a)_
 
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