Simplify StackMapStream encoding.

Test: test-art-host-gtest-stack_map_test
Change-Id: Ife021d03e4e486043ec609f9af8673ace7bde497
diff --git a/compiler/optimizing/code_generator.h b/compiler/optimizing/code_generator.h
index 3d58d29..e77d621 100644
--- a/compiler/optimizing/code_generator.h
+++ b/compiler/optimizing/code_generator.h
@@ -350,9 +350,7 @@
 
   void AddSlowPath(SlowPathCode* slow_path);
 
-  void BuildStackMaps(MemoryRegion stack_map_region,
-                      const DexFile::CodeItem* code_item_for_osr_check);
-  void ComputeStackMapSize(size_t* stack_map_size);
+  ScopedArenaVector<uint8_t> BuildStackMaps(const DexFile::CodeItem* code_item_for_osr_check);
   size_t GetNumberOfJitRoots() const;
 
   // Fills the `literals` array with literals collected during code generation.