[base] Use thread safe localtime_r() instead of localtime().

src/base/platform/platform-posix.cc

Index: src/base/platform/platform-posix.cc
diff --git a/src/base/platform/platform-posix.cc b/src/base/platform/platform-posix.cc
index bb340ab5f95af47f8c455e91ceeb1c182c8d7434..acc6cf33e9bd37a309a05d42ab73f56064b87182 100644
--- a/src/base/platform/platform-posix.cc
+++ b/src/base/platform/platform-posix.cc
@@ -398,7 +398,8 @@ void OS::ClearTimezoneCache(TimezoneCache* cache) {
 double OS::DaylightSavingsOffset(double time, TimezoneCache*) {
   if (std::isnan(time)) return std::numeric_limits<double>::quiet_NaN();
   time_t tv = static_cast<time_t>(std::floor(time/msPerSecond));
-  struct tm* t = localtime(&tv);  // NOLINT(runtime/threadsafe_fn)
+  struct tm tm;
+  struct tm* t = localtime_r(&tv, &tm);
   if (NULL == t) return std::numeric_limits<double>::quiet_NaN();
   return t->tm_isdst > 0 ? 3600 * msPerSecond : 0;
 }