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

runtime/vm/os_linux.cc

Index: runtime/vm/os_linux.cc
diff --git a/runtime/vm/os_linux.cc b/runtime/vm/os_linux.cc
index 2c27fd6ed769569b2e1cb9a70588c869d5c4dd88..ff857cbe3519b799f551b6bb69bccfd665247a54 100644
--- a/runtime/vm/os_linux.cc
+++ b/runtime/vm/os_linux.cc
@@ -59,6 +59,27 @@ 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;
 }