Change zygote_creation_lock_ to be member instead of static.

Static variables aren't thread safe and could cause the zygote to be
created twice.

Bug: 15133494
Change-Id: I65c8f089bed8de93f895b62b3dcff4c936931860
diff --git a/runtime/gc/heap.h b/runtime/gc/heap.h
index 6fe0dcf..890332a 100644
--- a/runtime/gc/heap.h
+++ b/runtime/gc/heap.h
@@ -778,6 +778,9 @@
   // useful for benchmarking since it reduces time spent in GC to a low %.
   const bool ignore_max_footprint_;
 
+  // Lock which guards zygote space creation.
+  Mutex zygote_creation_lock_;
+
   // If we have a zygote space.
   bool have_zygote_space_;