Alternative implementation would be to have something similar to what mojo currently has. A shared memory in SerializedHandle would be represented by a PlatformSharedMemoryRegion but callers will be still able to wrap/unwrap SharedMemoryHandle's.

Note this may overlap a bit with crbug.com/844456, as BitstreamBuffers interact with ppapi as well. See the links in that bug. I'm still investigating.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/1d004107b1a503bf971a246337515a4b35aca2dd

commit 1d004107b1a503bf971a246337515a4b35aca2dd
Author: Alexandr Ilin <alexilin@chromium.org>
Date: Wed May 30 10:55:32 2018

ppapi: Make SerializedHandle class move-only

This CL deletes the copy constructor and the copy assignment operator from the
ppapi::proxy::SerializedHandle class replacing them with move operations. This
is preparatory step to make SerializedHandle using the new shared memory
classes that are move-only.

SerializedHandle holds a system resource that should be properly closed after
use. Move semantics allow to introduce the more clear ownership model and
prevent resource leaks.

Bug:  845985 
Change-Id: Ie7202d18f4f0396133c53ee0faee111e07c58ee8
Reviewed-on: https://chromium-review.googlesource.com/1073200
Reviewed-by: Derek Schuff <dschuff@chromium.org>
Reviewed-by: Bill Budge <bbudge@chromium.org>
Commit-Queue: Alexandr Ilin <alexilin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#562796}
[modify] https://crrev.com/1d004107b1a503bf971a246337515a4b35aca2dd/components/nacl/loader/nacl_ipc_adapter.cc
[modify] https://crrev.com/1d004107b1a503bf971a246337515a4b35aca2dd/content/browser/renderer_host/pepper/pepper_file_io_host.cc
[modify] https://crrev.com/1d004107b1a503bf971a246337515a4b35aca2dd/content/renderer/pepper/pepper_audio_input_host.cc
[modify] https://crrev.com/1d004107b1a503bf971a246337515a4b35aca2dd/content/renderer/pepper/pepper_audio_output_host.cc
[modify] https://crrev.com/1d004107b1a503bf971a246337515a4b35aca2dd/content/renderer/pepper/pepper_media_stream_track_host_base.cc
[modify] https://crrev.com/1d004107b1a503bf971a246337515a4b35aca2dd/content/renderer/pepper/pepper_video_decoder_host.cc
[modify] https://crrev.com/1d004107b1a503bf971a246337515a4b35aca2dd/content/renderer/pepper/pepper_video_encoder_host.cc
[modify] https://crrev.com/1d004107b1a503bf971a246337515a4b35aca2dd/content/renderer/pepper/pepper_video_source_host.cc
[modify] https://crrev.com/1d004107b1a503bf971a246337515a4b35aca2dd/ppapi/host/ppapi_host.cc
[modify] https://crrev.com/1d004107b1a503bf971a246337515a4b35aca2dd/ppapi/host/ppapi_host.h
[modify] https://crrev.com/1d004107b1a503bf971a246337515a4b35aca2dd/ppapi/proxy/nacl_message_scanner.cc
[modify] https://crrev.com/1d004107b1a503bf971a246337515a4b35aca2dd/ppapi/proxy/resource_message_params.cc
[modify] https://crrev.com/1d004107b1a503bf971a246337515a4b35aca2dd/ppapi/proxy/resource_message_params.h
[modify] https://crrev.com/1d004107b1a503bf971a246337515a4b35aca2dd/ppapi/proxy/resource_message_test_sink.cc
[modify] https://crrev.com/1d004107b1a503bf971a246337515a4b35aca2dd/ppapi/proxy/resource_message_test_sink.h
[modify] https://crrev.com/1d004107b1a503bf971a246337515a4b35aca2dd/ppapi/proxy/serialized_handle.cc
[modify] https://crrev.com/1d004107b1a503bf971a246337515a4b35aca2dd/ppapi/proxy/serialized_handle.h
[modify] https://crrev.com/1d004107b1a503bf971a246337515a4b35aca2dd/ppapi/proxy/video_decoder_resource_unittest.cc
[modify] https://crrev.com/1d004107b1a503bf971a246337515a4b35aca2dd/ppapi/proxy/video_encoder_resource_unittest.cc

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/ebab9dadca758f4d3f0e0412a108cfc85a228536

commit ebab9dadca758f4d3f0e0412a108cfc85a228536
Author: Alexandr Ilin <alexilin@chromium.org>
Date: Fri Jun 01 02:37:47 2018

ppapi: Add SHARED_MEMORY_REGION type to SerializedHandle

This CL adds a new SHARED_MEMORY_REGION type to the SerializedHandle. This new
type is backed up by the base::subtle::PlatformSharedMemoryRegion class that
provides a new recommended API for the shared memory in Chrome.
Eventually, all uses of the SHARED_MEMORY type should be converted to the
SHARED_MEMORY_REGION.

Bug:  845985 
Change-Id: Icfb4605dadb6f111efe953d8b940cc879fa40763
Reviewed-on: https://chromium-review.googlesource.com/1076334
Reviewed-by: Ken Rockot <rockot@chromium.org>
Reviewed-by: Bill Budge <bbudge@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Derek Schuff <dschuff@chromium.org>
Commit-Queue: Alexandr Ilin <alexilin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#563500}
[modify] https://crrev.com/ebab9dadca758f4d3f0e0412a108cfc85a228536/components/nacl/loader/nacl_ipc_adapter.cc
[modify] https://crrev.com/ebab9dadca758f4d3f0e0412a108cfc85a228536/ipc/ipc_message_utils.cc
[modify] https://crrev.com/ebab9dadca758f4d3f0e0412a108cfc85a228536/ppapi/proxy/nacl_message_scanner.cc
[modify] https://crrev.com/ebab9dadca758f4d3f0e0412a108cfc85a228536/ppapi/proxy/ppapi_param_traits.cc
[modify] https://crrev.com/ebab9dadca758f4d3f0e0412a108cfc85a228536/ppapi/proxy/serialized_handle.cc
[modify] https://crrev.com/ebab9dadca758f4d3f0e0412a108cfc85a228536/ppapi/proxy/serialized_handle.h

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/c7d975fa69a1cbbda32b8a8a48a40305f3eb70ff

commit c7d975fa69a1cbbda32b8a8a48a40305f3eb70ff
Author: Alexandr Ilin <alexilin@chromium.org>
Date: Fri Jun 01 09:25:41 2018

ppapi: Add methods for sharing new shared memory classes with remote

This CL adds two new methods, ShareUnsafeSharedMemoryRegionWithRemote() and
ShareReadOnlySharedMemoryRegionWithRemote() to
ppapi::proxy::ProxyChannel::Delegate and to content::RendererPpapiHost classes
and all implementation classes.

These new methods are similar to the SharedSharedMemoryHandleWithRemote()
method in those classes but are supposed to work with the new shared memory
API.

Bug:  845985 
Change-Id: I8ccd92a1c81dca9495c85119e9a4e40fd6b69abf
Reviewed-on: https://chromium-review.googlesource.com/1078847
Commit-Queue: Alexandr Ilin <alexilin@chromium.org>
Reviewed-by: Antoine Labour <piman@chromium.org>
Reviewed-by: Raymes Khoury <raymes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#563571}
[modify] https://crrev.com/c7d975fa69a1cbbda32b8a8a48a40305f3eb70ff/components/pdf/renderer/pdf_accessibility_tree_browsertest.cc
[modify] https://crrev.com/c7d975fa69a1cbbda32b8a8a48a40305f3eb70ff/content/ppapi_plugin/ppapi_thread.cc
[modify] https://crrev.com/c7d975fa69a1cbbda32b8a8a48a40305f3eb70ff/content/ppapi_plugin/ppapi_thread.h
[modify] https://crrev.com/c7d975fa69a1cbbda32b8a8a48a40305f3eb70ff/content/public/renderer/renderer_ppapi_host.h
[modify] https://crrev.com/c7d975fa69a1cbbda32b8a8a48a40305f3eb70ff/content/renderer/pepper/mock_renderer_ppapi_host.cc
[modify] https://crrev.com/c7d975fa69a1cbbda32b8a8a48a40305f3eb70ff/content/renderer/pepper/mock_renderer_ppapi_host.h
[modify] https://crrev.com/c7d975fa69a1cbbda32b8a8a48a40305f3eb70ff/content/renderer/pepper/pepper_proxy_channel_delegate_impl.cc
[modify] https://crrev.com/c7d975fa69a1cbbda32b8a8a48a40305f3eb70ff/content/renderer/pepper/pepper_proxy_channel_delegate_impl.h
[modify] https://crrev.com/c7d975fa69a1cbbda32b8a8a48a40305f3eb70ff/content/renderer/pepper/renderer_ppapi_host_impl.cc
[modify] https://crrev.com/c7d975fa69a1cbbda32b8a8a48a40305f3eb70ff/content/renderer/pepper/renderer_ppapi_host_impl.h
[modify] https://crrev.com/c7d975fa69a1cbbda32b8a8a48a40305f3eb70ff/ppapi/nacl_irt/ppapi_dispatcher.cc
[modify] https://crrev.com/c7d975fa69a1cbbda32b8a8a48a40305f3eb70ff/ppapi/nacl_irt/ppapi_dispatcher.h
[modify] https://crrev.com/c7d975fa69a1cbbda32b8a8a48a40305f3eb70ff/ppapi/proxy/ppapi_proxy_test.cc
[modify] https://crrev.com/c7d975fa69a1cbbda32b8a8a48a40305f3eb70ff/ppapi/proxy/ppapi_proxy_test.h
[modify] https://crrev.com/c7d975fa69a1cbbda32b8a8a48a40305f3eb70ff/ppapi/proxy/proxy_channel.cc
[modify] https://crrev.com/c7d975fa69a1cbbda32b8a8a48a40305f3eb70ff/ppapi/proxy/proxy_channel.h

It's fixed now. Now we need to remove all usages of SerializedHandle::Type::SHARED_MEMORY. Different use-cases are tracked separately.