Move sync APIs out of "Worker" into "SyncWorker"

Over in ServiceWorker we want to get all of the APIs available to workers
(FormData, indexeddb, URL etc), but not the synchronous interfaces such as
http://dev.w3.org/2006/webapi/FileAPI/#readingOnThreads

We could expose each API on ServiceWorker individually, but other
multi-client worker contexts are likely to want to do the same. It could be
argued that filesystem's *Sync methods shouldn't exist in a SharedWorker
scope.

Would it makes sense to do one of the following?:

* Expose the sync methods on DedicatedWorker only - this would require
usage stats of sync methods in SharedWorkers to be insignificant
* Expose the sync methods on a new global SyncWorker, where
DedicatedWorkerGlobalScope would
be [Global=Worker,SyncWorker,DedicatedWorker], SharedWorkerGlobalScope
would be [Global=Worker,SyncWorker,SharedWorker]