Add ThreadBarrier; use in GCMarker and unit test

runtime/vm/random.cc

Index: runtime/vm/random.cc
diff --git a/runtime/vm/random.cc b/runtime/vm/random.cc
index 18703d0d315f16e6a24e2e5ce0625d121d2793fa..6f1b6eba17641f7c9064a7efacef96979622cc67 100644
--- a/runtime/vm/random.cc
+++ b/runtime/vm/random.cc
@@ -25,6 +25,12 @@ Random::Random() {
     // We did not get a seed so far. As a fallback we do use the current time.
     seed = OS::GetCurrentTimeMicros();
   }
+  Initialize(seed);
+}
+
+
+void Random::Initialize(uint64_t seed) {
+  ASSERT(seed != 0);
   // Crank the next state a couple of times.
   _state = seed;
   NextState();
@@ -33,6 +39,12 @@ Random::Random() {
   NextState();
 }
 
+
+Random::Random(uint64_t seed) {
+  Initialize(seed);
+}
+
+
 Random::~Random() {
   // Nothing to be done here.
 }