Move ArtFields and ArtMethods to be a length prefixed array

Fixes race conditions between changing method and fields arrays
being seen in the wrong order by the GC.

Bug: 22832610
Change-Id: Ia21d6698f73ba207a6392c3d6b9be2658933073f
diff --git a/patchoat/patchoat.h b/patchoat/patchoat.h
index 466dacb..43cdaea 100644
--- a/patchoat/patchoat.h
+++ b/patchoat/patchoat.h
@@ -207,6 +207,8 @@
   TimingLogger* timings_;
 
   friend class FixupRootVisitor;
+  friend class PatchOatArtFieldVisitor;
+  friend class PatchOatArtMethodVisitor;
   DISALLOW_IMPLICIT_CONSTRUCTORS(PatchOat);
 };