Subspaces are Heap data structures to begin with, and this refactoring is needed in preparation for the global GC.
<rdar://problem/85875751>
Created attachment 445469 [details] proposed patch.
Comment on attachment 445469 [details] proposed patch. r=me
Created attachment 445472 [details] patch for landing. Thanks for the review. This patch also includes a fix for the watchOS EWS build failure.
Created attachment 445484 [details] patch for landing + speculative fix for Windows build.
Landed in r286347: <http://trac.webkit.org/r286347>.