Stop using StringPiece::ToString().

Change-Id: Ia03c4bcce55d2784df9c4527e56bd5d4129cc9d5
Reviewed-on: https://code-review.googlesource.com/23010
Reviewed-by: Paul Wankadia <[email protected]>
diff --git a/re2/testing/string_generator_test.cc b/re2/testing/string_generator_test.cc
index 1f10a80..2c040a3 100644
--- a/re2/testing/string_generator_test.cc
+++ b/re2/testing/string_generator_test.cc
@@ -47,7 +47,7 @@
   }
 
   while (g.HasNext()) {
-    string s = g.Next().ToString();
+    string s = string(g.Next());
     n++;
 
     // Check that all characters in s appear in alphabet.