Set the stack base value for thread pool threads at the start so that profile ticks for threads show up even if it never runs Dart code.

R=rmacnak@google.com

Review URL: https://codereview.chromium.org/1815533002 .

runtime/vm/thread.cc

@@ -331,6 +331,7 @@ void Thread::ClearStackLimit() {
 uword Thread::GetCurrentStackPointer() {
   // Since AddressSanitizer's detect_stack_use_after_return instruments the
   // C++ code to give out fake stack addresses, we call a stub in that case.
+  ASSERT(StubCode::GetStackPointer_entry() != NULL);
   uword (*func)() = reinterpret_cast<uword (*)()>(
       StubCode::GetStackPointer_entry()->EntryPoint());
   // But for performance (and to support simulators), we normally use a local.

runtime/vm/thread_pool.cc

@@ -433,6 +433,9 @@ void ThreadPool::Worker::Main(uword args) {
   ThreadJoinId join_id = os_thread->join_id();
   ThreadPool* pool;
 
+  // Set the thread's stack_base based on the current stack pointer.
+  os_thread->set_stack_base(Thread::GetCurrentStackPointer());
+
   {
     MonitorLocker ml(&worker->monitor_);
     ASSERT(worker->task_);