Code: void MojoAudioOutputIPC::StreamCreated(
    mojo::ScopedSharedBufferHandle shared_memory,
    mojo::ScopedHandle socket) {
  DCHECK_CALLED_ON_VALID_THREAD(thread_checker_);
  DCHECK(delegate_);
  DCHECK(socket.is_valid());
  DCHECK(shared_memory.is_valid());

  base::PlatformFile socket_handle;
  auto result = mojo::UnwrapPlatformFile(std::move(socket), &socket_handle);
   DCHECK_EQ(result, MOJO_RESULT_OK);
 
   base::SharedMemoryHandle memory_handle;
  bool read_only = false;
   size_t memory_length = 0;
   result = mojo::UnwrapSharedMemoryHandle(
      std::move(shared_memory), &memory_handle, &memory_length, &read_only);
   DCHECK_EQ(result, MOJO_RESULT_OK);
  DCHECK(!read_only);
 
   delegate_->OnStreamCreated(memory_handle, socket_handle);
 }

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
void MojoAudioOutputIPC::StreamCreated(
    mojo::ScopedSharedBufferHandle shared_memory,
    mojo::ScopedHandle socket) {
  DCHECK_CALLED_ON_VALID_THREAD(thread_checker_);
  DCHECK(delegate_);
  DCHECK(socket.is_valid());
  DCHECK(shared_memory.is_valid());

  base::PlatformFile socket_handle;
  auto result = mojo::UnwrapPlatformFile(std::move(socket), &socket_handle);
   DCHECK_EQ(result, MOJO_RESULT_OK);
 
   base::SharedMemoryHandle memory_handle;
  mojo::UnwrappedSharedMemoryHandleProtection protection;
   size_t memory_length = 0;
   result = mojo::UnwrapSharedMemoryHandle(
      std::move(shared_memory), &memory_handle, &memory_length, &protection);
   DCHECK_EQ(result, MOJO_RESULT_OK);
  DCHECK_EQ(protection,
            mojo::UnwrappedSharedMemoryHandleProtection::kReadWrite);
 
   delegate_->OnStreamCreated(memory_handle, socket_handle);
 }
