Add equals to C++ String

This is a baby step towards using String instead of StringPiece.

Change-Id: I34baa1c91e99f78151ec1fa9175eb5df30fa0292
diff --git a/src/class_linker.h b/src/class_linker.h
index d6c2f0c..b504095 100644
--- a/src/class_linker.h
+++ b/src/class_linker.h
@@ -206,6 +206,7 @@
   FRIEND_TEST(DexCacheTest, Open);
   friend class ObjectTest;
   FRIEND_TEST(ObjectTest, AllocObjectArray);
+  FRIEND_TEST(ObjectTest, StringEquals);
   DISALLOW_COPY_AND_ASSIGN(ClassLinker);
 };