Implements JS bindings for mojo shared buffer.

third_party/WebKit/Source/core/mojo/MojoSharedBuffer.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.
+
+[
+    RuntimeEnabled=MojoJS,
+] interface MojoSharedBuffer {
+    readonly attribute ArrayBuffer buffer;

[jbroman, 2017/02/27 20:14:44]

Can this be mapped in multiple threads/processes at once? (i.e., do we need to
think about whether SharedArrayBuffer is necessary here, and what promises V8
does or does not make about when writes are flushed?)

[alokp, 2017/02/27 20:30:34]

Now that I think about it, I named this interface wrong. It should instead be
called MojoMappedBuffer. It does not represent the shared buffer but a memory
region in the shared buffer.

I do not think SharedArrayBuffer is applicable here. Mojo core layer should take
care of read/write synchronization between multiple mappings into a common
buffer. yzshen@: Is my understanding correct?

[yzshen1, 2017/02/27 21:05:27]

I don't think mojo provides any extra read/write synchronization on top of OS
shared memory API.

+    void unmap();
+};