Refactor large object sweeping.

Moved basic sweeping logic into large_object_space.cc.
Renamed SpaceSetMap -> ObjectSet.

Change-Id: I938c1f29f69b0682350347da2bd5de021c0e0224
diff --git a/runtime/gc/space/space.h b/runtime/gc/space/space.h
index 31bbb7b..5292344 100644
--- a/runtime/gc/space/space.h
+++ b/runtime/gc/space/space.h
@@ -314,11 +314,11 @@
 // is suitable for use for large primitive arrays.
 class DiscontinuousSpace : public Space {
  public:
-  accounting::SpaceSetMap* GetLiveObjects() const {
+  accounting::ObjectSet* GetLiveObjects() const {
     return live_objects_.get();
   }
 
-  accounting::SpaceSetMap* GetMarkObjects() const {
+  accounting::ObjectSet* GetMarkObjects() const {
     return mark_objects_.get();
   }
 
@@ -331,8 +331,8 @@
  protected:
   DiscontinuousSpace(const std::string& name, GcRetentionPolicy gc_retention_policy);
 
-  UniquePtr<accounting::SpaceSetMap> live_objects_;
-  UniquePtr<accounting::SpaceSetMap> mark_objects_;
+  UniquePtr<accounting::ObjectSet> live_objects_;
+  UniquePtr<accounting::ObjectSet> mark_objects_;
 
  private:
   DISALLOW_COPY_AND_ASSIGN(DiscontinuousSpace);