Rename Thread -> OSThread.

It's a collection of static utility methods for primitive operations on OS threads.

Make room in the namespace for upcoming class that will represent all the complex state of a VM thread.

Review URL: https://codereview.chromium.org//796063006

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge/dart@42895 260f80e4-7a28-3924-810f-c04153c831b5

runtime/vm/message_handler_test.cc

@@ -311,7 +311,7 @@ UNIT_TEST_CASE(MessageHandler_Run) {
   info.handler = &handler;
   info.ports = ports;
   info.count = 10;
-  Thread::Start(SendMessages, reinterpret_cast<uword>(&info));
+  OSThread::Start(SendMessages, reinterpret_cast<uword>(&info));
   while (sleep < kMaxSleep && handler.message_count() < 11) {
     OS::Sleep(10);
     sleep += 10;