Implements JS bindings for mojo shared buffer.

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

 // 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.
 
 callback MojoWatchCallback = void (MojoResult result);
 
 [
     RuntimeEnabled=MojoJS,
 ] interface MojoHandle {
     void close();
     [CallWith=ScriptState] MojoWatcher watch(MojoHandleSignals signals, MojoWatchCallback callback);
+
+    // TODO(alokp): Create MessagePipeHandle, a subclass of MojoHandle
+    // and move the following member functions.
     MojoResult writeMessage(BufferSource buffer, sequence<MojoHandle> handles);
     MojoReadMessageResult readMessage(optional MojoReadMessageFlags flags);
+
+    // TODO(alokp): Create SharedBufferHandle, a subclass of MojoHandle
+    // and move the following member functions.
+    MojoMapBufferResult mapBuffer(unsigned long offset, unsigned long numBytes);
 };