[presubmit] Enable runtime/threadsafe_fn linter checking.

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 8a037c02d335eeb8a961072b5749825a6040568c..046dbb69c30f03f50a70d81114f60fd58173bb80 100644
--- a/src/base/platform/platform-posix.cc
+++ b/src/base/platform/platform-posix.cc
@@ -384,7 +384,7 @@ 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);
+  struct tm* t = localtime(&tv);  // NOLINT(runtime/threadsafe_fn)
   if (NULL == t) return std::numeric_limits<double>::quiet_NaN();
   return t->tm_isdst > 0 ? 3600 * msPerSecond : 0;
 }