Implement a poor man's PostAfterStartupTask() function.

base/synchronization/cancellation_flag.cc

Index: base/synchronization/cancellation_flag.cc
diff --git a/base/synchronization/cancellation_flag.cc b/base/synchronization/cancellation_flag.cc
index ad3b551169d481e7715f5238ab9a4d81dbe2928a..9cd6301c3828af0f7f71143a37923d62e256b1f6 100644
--- a/base/synchronization/cancellation_flag.cc
+++ b/base/synchronization/cancellation_flag.cc
@@ -19,4 +19,8 @@ bool CancellationFlag::IsSet() const {
   return base::subtle::Acquire_Load(&flag_) != 0;
 }
 
+void CancellationFlag::UnsafeReset() {
+  base::subtle::Release_Store(&flag_, 0);
+}
+
 }  // namespace base