Fix cpplint runtime/explicit issues

Change-Id: I352ba0b427f1ff9b22887693952b180eae0839ba
diff --git a/compiler/dex/arena_allocator.h b/compiler/dex/arena_allocator.h
index cc81e50..0ad859e 100644
--- a/compiler/dex/arena_allocator.h
+++ b/compiler/dex/arena_allocator.h
@@ -47,7 +47,7 @@
       kNumAllocKinds
     };
 
-  ArenaAllocator(size_t default_size = ARENA_DEFAULT_BLOCK_SIZE);
+  explicit ArenaAllocator(size_t default_size = ARENA_DEFAULT_BLOCK_SIZE);
   ~ArenaAllocator();
   void* NewMem(size_t size, bool zero, ArenaAllocKind kind);
   size_t BytesAllocated() {
@@ -86,7 +86,7 @@
      void Dump(std::ostream& os) const {
        arena_.DumpMemStats(os);
      }
-     MemStats(const ArenaAllocator &arena) : arena_(arena) {};
+     explicit MemStats(const ArenaAllocator &arena) : arena_(arena) {};
   private:
     const ArenaAllocator &arena_;
 }; // MemStats