Add a rand_util method for generating a random string.

base/rand_util.h

Index: base/rand_util.h
===================================================================
--- base/rand_util.h	(revision 83409)
+++ base/rand_util.h	(working copy)
@@ -6,6 +6,8 @@
 #define BASE_RAND_UTIL_H_
 #pragma once
 
+#include <string>
+
 #include "base/base_api.h"
 #include "base/basictypes.h"
 
@@ -27,6 +29,9 @@
 // Returns a random double in range [0, 1). Thread-safe.
 BASE_API double RandDouble();
 
+// Returns a random string of the specified length.
+BASE_API std::string RandBytesAsString(size_t length);
+
 }  // namespace base
 
 #endif  // BASE_RAND_UTIL_H_