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 CommandBufferProxyImpl::OnGetGpuFenceHandleComplete(
    uint32_t gpu_fence_id,
    const gfx::GpuFenceHandle& handle) {
  auto gpu_fence = base::MakeUnique<gfx::GpuFence>(handle);

  GetGpuFenceTaskMap::iterator it = get_gpu_fence_tasks_.find(gpu_fence_id);
  if (it == get_gpu_fence_tasks_.end()) {
    DLOG(ERROR) << "GPU process sent invalid GetGpuFenceHandle response.";
    base::AutoLock lock(last_state_lock_);
    OnGpuAsyncMessageError(gpu::error::kInvalidGpuMessage,
                           gpu::error::kLostContext);
    return;
  }
  auto callback = std::move(it->second);
  get_gpu_fence_tasks_.erase(it);
  std::move(callback).Run(std::move(gpu_fence));
}

void CommandBufferProxyImpl::OnGetGpuFenceHandleComplete(
    uint32_t gpu_fence_id,
    const gfx::GpuFenceHandle& handle) {
  auto gpu_fence = base::MakeUnique<gfx::GpuFence>(handle);

  GetGpuFenceTaskMap::iterator it = get_gpu_fence_tasks_.find(gpu_fence_id);
  if (it == get_gpu_fence_tasks_.end()) {
    DLOG(ERROR) << "GPU process sent invalid GetGpuFenceHandle response.";
    base::AutoLock lock(last_state_lock_);
    OnGpuAsyncMessageError(gpu::error::kInvalidGpuMessage,
                           gpu::error::kLostContext);
    return;
  }
  auto callback = std::move(it->second);
  get_gpu_fence_tasks_.erase(it);
  std::move(callback).Run(std::move(gpu_fence));
}
