Reapply "Add support for timezone offset and timezone name."

runtime/vm/os_macos.cc

Index: runtime/vm/os_macos.cc
diff --git a/runtime/vm/os_macos.cc b/runtime/vm/os_macos.cc
index d602485892e546c89b04c9053d7ac9ad6f208b85..3936e99e93ea46546e6623ca1749ec72d25977cc 100644
--- a/runtime/vm/os_macos.cc
+++ b/runtime/vm/os_macos.cc
@@ -60,6 +60,27 @@ bool OS::MkTime(tm* tm, int64_t* seconds_result) {
 }
 
 
+bool OS::GetTimeZoneName(int64_t seconds_since_epoch,
+                         const char** name_result) {
+  tm decomposed;
+  bool succeeded = LocalTime(seconds_since_epoch, &decomposed);
+  if (!succeeded) return false;
+  *name_result = decomposed.tm_zone;
+  return true;
+}
+
+
+bool OS::GetTimeZoneOffsetInSeconds(int64_t seconds_since_epoch,
+                                    int* offset_result) {
+  tm decomposed;
+  bool succeeded = LocalTime(seconds_since_epoch, &decomposed);
+  if (!succeeded) return false;
+  // Even if the offset was 24 hours it would still easily fit into 32 bits.
+  *offset_result = static_cast<int>(decomposed.tm_gmtoff);
+  return true;
+}
+
+
 int64_t OS::GetCurrentTimeMillis() {
   return GetCurrentTimeMicros() / 1000;
 }