Mojo: Add a C++-style SharedBuffer API.

mojo/public/cpp/system/buffer.cc

+// Copyright 2016 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.
+
+#include "mojo/public/cpp/system/buffer.h"
+
+namespace mojo {
+
+ScopedSharedBufferHandle SharedBufferHandle::Clone(bool read_only) const {

[Ken Rockot(use gerrit already), 2016/06/03 20:14:35]

optional nit: Maybe add an enum in SharedBufferHandle:

  enum class AccessMode {
    READ_WRITE,
    READ_ONLY
  };

And use that instead of a bool? Easier to read call sites.

[Sam McNally, 2016/06/05 23:49:20]

Done.

+  ScopedSharedBufferHandle result;
+  if (!is_valid())
+    return result;
+
+  MojoDuplicateBufferHandleOptions options = {
+      sizeof(options), MOJO_DUPLICATE_BUFFER_HANDLE_OPTIONS_FLAG_NONE};
+  if (read_only)
+    options.flags |= MOJO_DUPLICATE_BUFFER_HANDLE_OPTIONS_FLAG_READ_ONLY;
+  SharedBufferHandle result_handle;
+  MojoDuplicateBufferHandle(value(), &options, result_handle.mutable_value());
+  result.reset(result_handle);
+  return result;
+}
+
+ScopedSharedBufferMapping SharedBufferHandle::Map(uint64_t size) const {
+  return MapAtOffset(size, 0);
+}
+
+ScopedSharedBufferMapping SharedBufferHandle::MapAtOffset(
+    uint64_t size,
+    uint64_t offset) const {
+  void* buffer = nullptr;
+  MojoMapBuffer(value(), offset, size, &buffer, MOJO_MAP_BUFFER_FLAG_NONE);
+  return ScopedSharedBufferMapping(buffer);
+}
+
+}  // namespace mojo