Add Chromium-style TimeDelta, Time and TimeTicks classes, and a new ElapsedTimer class.

src/platform-macos.cc

Index: src/platform-macos.cc
diff --git a/src/platform-macos.cc b/src/platform-macos.cc
index 6135cd13740f03bb15f62ddfb26e4c80ae83bea8..9d8d076bb052331c10571eb55103755f4fb96ef2 100644
--- a/src/platform-macos.cc
+++ b/src/platform-macos.cc
@@ -441,7 +441,7 @@ Semaphore* OS::CreateSemaphore(int count) {
 
 void OS::SetUp() {
   // Seed the random number generator. We preserve microsecond resolution.
-  uint64_t seed = Ticks() ^ (getpid() << 16);
+  uint64_t seed = static_cast<uint64_t>(TimeCurrentMillis()) ^ (getpid() << 16);
   srandom(static_cast<unsigned int>(seed));
   limit_mutex = CreateMutex();
 }