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/Android.mk b/runtime/Android.mk
index 9e6d5f9..530488e 100644
--- a/runtime/Android.mk
+++ b/runtime/Android.mk
@@ -65,6 +65,7 @@
 	gc/space/malloc_space.cc \
 	gc/space/rosalloc_space.cc \
 	gc/space/space.cc \
+	gc/space/zygote_space.cc \
 	hprof/hprof.cc \
 	image.cc \
 	indirect_reference_table.cc \