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}
void VideoCaptureImpl::OnBufferDestroyed(int32_t buffer_id) {
  DCHECK(io_thread_checker_.CalledOnValidThread());

  const auto& cb_iter = client_buffers_.find(buffer_id);
  if (cb_iter != client_buffers_.end()) {
    DCHECK(!cb_iter->second.get() || cb_iter->second->HasOneRef())
        << "Instructed to delete buffer we are still using.";
    client_buffers_.erase(cb_iter);
  }
}

void VideoCaptureImpl::OnBufferDestroyed(int32_t buffer_id) {
  DCHECK(io_thread_checker_.CalledOnValidThread());

  const auto& cb_iter = client_buffers_.find(buffer_id);
  if (cb_iter != client_buffers_.end()) {
    DCHECK(!cb_iter->second.get() || cb_iter->second->HasOneRef())
        << "Instructed to delete buffer we are still using.";
    client_buffers_.erase(cb_iter);
  }
}
