Implements JS bindings for mojo shared buffer.

third_party/WebKit/Source/core/mojo/MojoSharedBufferMapping.idl

Index: third_party/WebKit/Source/core/mojo/MojoSharedBufferMapping.idl
diff --git a/third_party/WebKit/Source/core/mojo/MojoSharedBufferMapping.idl b/third_party/WebKit/Source/core/mojo/MojoSharedBufferMapping.idl
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+++ b/third_party/WebKit/Source/core/mojo/MojoSharedBufferMapping.idl
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+// Copyright 2017 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+[
+    RuntimeEnabled=MojoJS,
+] interface MojoSharedBufferMapping {
+    readonly attribute ArrayBuffer buffer;
+    void unmap();
+};