Code: CommandBufferProxyImpl::AllocateAndMapSharedMemory(size_t size) {
  mojo::ScopedSharedBufferHandle handle =
      mojo::SharedBufferHandle::Create(size);
  if (!handle.is_valid()) {
    DLOG(ERROR) << "AllocateAndMapSharedMemory: Create failed";
    return nullptr;
  }
 
   base::SharedMemoryHandle platform_handle;
   size_t shared_memory_size;
  bool readonly;
   MojoResult result = mojo::UnwrapSharedMemoryHandle(
      std::move(handle), &platform_handle, &shared_memory_size, &readonly);
   if (result != MOJO_RESULT_OK) {
     DLOG(ERROR) << "AllocateAndMapSharedMemory: Unwrap failed";
     return nullptr;
   }
   DCHECK_EQ(shared_memory_size, size);
 
  auto shm = std::make_unique<base::SharedMemory>(platform_handle, readonly);
   if (!shm->Map(size)) {
     DLOG(ERROR) << "AllocateAndMapSharedMemory: Map failed";
     return nullptr;
  }

  return shm;
}

Commit Message: 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}
CWE ID: CWE-787
CommandBufferProxyImpl::AllocateAndMapSharedMemory(size_t size) {
  mojo::ScopedSharedBufferHandle handle =
      mojo::SharedBufferHandle::Create(size);
  if (!handle.is_valid()) {
    DLOG(ERROR) << "AllocateAndMapSharedMemory: Create failed";
    return nullptr;
  }
 
   base::SharedMemoryHandle platform_handle;
   size_t shared_memory_size;
  mojo::UnwrappedSharedMemoryHandleProtection protection;
   MojoResult result = mojo::UnwrapSharedMemoryHandle(
      std::move(handle), &platform_handle, &shared_memory_size, &protection);
   if (result != MOJO_RESULT_OK) {
     DLOG(ERROR) << "AllocateAndMapSharedMemory: Unwrap failed";
     return nullptr;
   }
   DCHECK_EQ(shared_memory_size, size);
 
  bool read_only =
      protection == mojo::UnwrappedSharedMemoryHandleProtection::kReadOnly;
  auto shm = std::make_unique<base::SharedMemory>(platform_handle, read_only);
   if (!shm->Map(size)) {
     DLOG(ERROR) << "AllocateAndMapSharedMemory: Map failed";
     return nullptr;
  }

  return shm;
}
