Add 10 minute random fuzz to automatic update check event schedule.

Schedules the initial check at 7 minutes and periodic checks at 45 minutes
with +/- 5 minute fuzz.

BUG=5983
TEST=unit tests, gmerged on device, inspected logs, observed checks

Change-Id: Ifd38574c4265fec78f6e330f965c2b1054b2dddb

Review URL: http://codereview.chromium.org/3173041
diff --git a/utils.cc b/utils.cc
index cf1c3f5..246bfaa 100644
--- a/utils.cc
+++ b/utils.cc
@@ -20,6 +20,7 @@
 
 #include "base/file_path.h"
 #include "base/file_util.h"
+#include "base/rand_util.h"
 #include "base/string_util.h"
 #include "base/logging.h"
 #include "update_engine/file_writer.h"
@@ -457,6 +458,12 @@
   return static_cast<int>(priority_rhs) - static_cast<int>(priority_lhs);
 }
 
+int FuzzInt(int value, unsigned int range) {
+  int min = value - range / 2;
+  int max = value + range - range / 2;
+  return base::RandInt(min, max);
+}
+
 const char* const kStatefulPartition = "/mnt/stateful_partition";
 
 }  // namespace utils