Correct mojo::WrapSharedMemoryHandle usage

Fixes some incorrect uses of mojo::WrapSharedMemoryHandle which
were assuming that the call actually has any control over the memory
protection applied to a handle when mapped.

Where fixing usage is infeasible for this CL, TODOs are added to
annotate follow-up work.

Also updates the API and documentation to (hopefully) improve clarity
and avoid similar mistakes from being made in the future.

BUG=792900

Cq-Include-Trybots: master.tryserver.chromium.android:android_optional_gpu_tests_rel;master.tryserver.chromium.linux:linux_optional_gpu_tests_rel;master.tryserver.chromium.mac:mac_optional_gpu_tests_rel;master.tryserver.chromium.win:win_optional_gpu_tests_rel
Change-Id: I0578aaa9ca3bfcb01aaf2451315d1ede95458477
Reviewed-on: https://chromium-review.googlesource.com/818282
Reviewed-by: Wei Li <weili@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Sadrul Chowdhury <sadrul@chromium.org>
Reviewed-by: Yuzhu Shen <yzshen@chromium.org>
Reviewed-by: Robert Sesek <rsesek@chromium.org>
Commit-Queue: Ken Rockot <rockot@chromium.org>
Cr-Commit-Position: refs/heads/master@{#530268}
    FromMojom(media::mojom::DecodeError error,
              media::JpegDecodeAccelerator::Error* out) {
  switch (error) {
    case media::mojom::DecodeError::NO_ERRORS:
      *out = media::JpegDecodeAccelerator::Error::NO_ERRORS;
      return true;
    case media::mojom::DecodeError::INVALID_ARGUMENT:
      *out = media::JpegDecodeAccelerator::Error::INVALID_ARGUMENT;
      return true;
    case media::mojom::DecodeError::UNREADABLE_INPUT:
      *out = media::JpegDecodeAccelerator::Error::UNREADABLE_INPUT;
      return true;
    case media::mojom::DecodeError::PARSE_JPEG_FAILED:
      *out = media::JpegDecodeAccelerator::Error::PARSE_JPEG_FAILED;
      return true;
    case media::mojom::DecodeError::UNSUPPORTED_JPEG:
      *out = media::JpegDecodeAccelerator::Error::UNSUPPORTED_JPEG;
      return true;
    case media::mojom::DecodeError::PLATFORM_FAILURE:
      *out = media::JpegDecodeAccelerator::Error::PLATFORM_FAILURE;
      return true;
  }
  NOTREACHED();
  return false;
}

    FromMojom(media::mojom::DecodeError error,
              media::JpegDecodeAccelerator::Error* out) {
  switch (error) {
    case media::mojom::DecodeError::NO_ERRORS:
      *out = media::JpegDecodeAccelerator::Error::NO_ERRORS;
      return true;
    case media::mojom::DecodeError::INVALID_ARGUMENT:
      *out = media::JpegDecodeAccelerator::Error::INVALID_ARGUMENT;
      return true;
    case media::mojom::DecodeError::UNREADABLE_INPUT:
      *out = media::JpegDecodeAccelerator::Error::UNREADABLE_INPUT;
      return true;
    case media::mojom::DecodeError::PARSE_JPEG_FAILED:
      *out = media::JpegDecodeAccelerator::Error::PARSE_JPEG_FAILED;
      return true;
    case media::mojom::DecodeError::UNSUPPORTED_JPEG:
      *out = media::JpegDecodeAccelerator::Error::UNSUPPORTED_JPEG;
      return true;
    case media::mojom::DecodeError::PLATFORM_FAILURE:
      *out = media::JpegDecodeAccelerator::Error::PLATFORM_FAILURE;
      return true;
  }
  NOTREACHED();
  return false;
}
