Add -Wshorten-64-to-32 flag to mac builds.

src/base/platform/time.cc

Index: src/base/platform/time.cc
diff --git a/src/base/platform/time.cc b/src/base/platform/time.cc
index b6a11cff34f313661536eb0d20b43cf7d598e479..7e4c8fc0ac902db962fa3b5a3337f1e876ebd561 100644
--- a/src/base/platform/time.cc
+++ b/src/base/platform/time.cc
@@ -111,7 +111,7 @@ TimeDelta TimeDelta::FromMachTimespec(struct mach_timespec ts) {
 struct mach_timespec TimeDelta::ToMachTimespec() const {
   struct mach_timespec ts;
   DCHECK(delta_ >= 0);
-  ts.tv_sec = delta_ / Time::kMicrosecondsPerSecond;
+  ts.tv_sec = static_cast<unsigned>(delta_ / Time::kMicrosecondsPerSecond);
   ts.tv_nsec = (delta_ % Time::kMicrosecondsPerSecond) *
       Time::kNanosecondsPerMicrosecond;
   return ts;