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 SimulateOnBufferCreated(int buffer_id, const base::SharedMemory& shm) {
     video_capture_impl_->OnBufferCreated(
        buffer_id, mojo::WrapSharedMemoryHandle(
                       shm.GetReadOnlyHandle(), shm.mapped_size(),
                       mojo::UnwrappedSharedMemoryHandleProtection::kReadOnly));
   }

   void SimulateOnBufferCreated(int buffer_id, const base::SharedMemory& shm) {
    auto handle = base::SharedMemory::DuplicateHandle(shm.handle());
     video_capture_impl_->OnBufferCreated(
        buffer_id, mojo::WrapSharedMemoryHandle(handle, shm.mapped_size(),
                                                true /* read_only */));
   }
