* Use clearer pref names.
* Default (and only support) IPDL dispatching.
* Make DispatchCommands async-only.
* Sync ipdl command per sync webgl entrypoint.
  * Eat the boilerplate cost, since there's not too many.
* Run SerializedSize off same path as Serialize.
* All shmem uploads go through normal DispatchCommands.
* Defer pruning of dead code for now so we can iterate quickly.
* Use Read/Write(begin,end) instead of (begin,size).
  * This would have prevented a bug where we read/wrote N*sizeof(T)*sizeof(T).

Differential Revision: https://phabricator.services.mozilla.com/D81495