Wrap `OsIpcSharedMemory` with Arc.

[sagudev]

The Clone implementation for OsIpcSharedMemory ¹ calls dup which seems to allocate a new file descriptor. Maybe making this hold an Arc internally would reduce the number of file descriptors that we allocate though.

Originally posted by @mrobinson in servo/servo#40136 (comment)

Arc::clone is probably cheaper then dup and we could possible reduce number of fds.

Can we avoid double arc without to much complexity in inprocess?

Footnotes

1. https://github.com/servo/ipc-channel/blob/main/src/platform/unix/mod.rs#L823 ↩

[mrobinson]

I think we should definitely try this.