Move varint encoding/decoding to BitMemoryReader/Writer.

Test: test-art-host-gtest-bit_memory_region_test
Change-Id: I0f7d7dfc9efe9df0e3e746b1741c46f06a0bd3b4
diff --git a/runtime/stack_map.h b/runtime/stack_map.h
index cde08f3..bfd646d 100644
--- a/runtime/stack_map.h
+++ b/runtime/stack_map.h
@@ -436,9 +436,9 @@
   ALWAYS_INLINE static QuickMethodFrameInfo DecodeFrameInfo(const uint8_t* data) {
     BitMemoryReader reader(data);
     return QuickMethodFrameInfo(
-        DecodeVarintBits(reader) * kStackAlignment,  // Decode packed_frame_size_ and unpack.
-        DecodeVarintBits(reader),  // core_spill_mask_.
-        DecodeVarintBits(reader));  // fp_spill_mask_.
+        reader.ReadVarint() * kStackAlignment,  // Decode packed_frame_size_ and unpack.
+        reader.ReadVarint(),  // core_spill_mask_.
+        reader.ReadVarint());  // fp_spill_mask_.
   }
 
  private: