Add zygote space as its own space type.

Helps prevent errors caused from doing invalid operations on the
old alloc space.

Removed some duplicated code in mark_sweep.cc and semi_space.cc.

Change-Id: I67a772cab30d698744c918aad581053f282a4a99
diff --git a/runtime/gc/space/rosalloc_space.cc b/runtime/gc/space/rosalloc_space.cc
index e5993f6..177e38e 100644
--- a/runtime/gc/space/rosalloc_space.cc
+++ b/runtime/gc/space/rosalloc_space.cc
@@ -312,6 +312,7 @@
 }
 
 void RosAllocSpace::Clear() {
+  // TODO: Delete and create new mspace here.
   madvise(GetMemMap()->Begin(), GetMemMap()->Size(), MADV_DONTNEED);
   GetLiveBitmap()->Clear();
   GetMarkBitmap()->Clear();