shill: replace rand() with std::default_random_engine

Replace rand() with STL random number generator to have
superior quality of random numbers, thread safety and
stop linter from complaining.

BUG=None
TEST=Unit tests pass.

Change-Id: I80da776c70bbd8bffe10916554393621ad60131e
Reviewed-on: https://chromium-review.googlesource.com/204430
Tested-by: Alex Vakulenko <avakulenko@chromium.org>
Reviewed-by: mukesh agrawal <quiche@chromium.org>
Commit-Queue: Alex Vakulenko <avakulenko@chromium.org>
diff --git a/default_profile.h b/default_profile.h
index 60125fa..afae425 100644
--- a/default_profile.h
+++ b/default_profile.h
@@ -5,6 +5,7 @@
 #ifndef SHILL_DEFAULT_PROFILE_H_
 #define SHILL_DEFAULT_PROFILE_H_
 
+#include <random>
 #include <string>
 #include <vector>
 
@@ -84,6 +85,7 @@
   const base::FilePath storage_path_;
   const std::string profile_id_;
   const Manager::Properties &props_;
+  std::default_random_engine random_engine_;
 
   DISALLOW_COPY_AND_ASSIGN(DefaultProfile);
 };