Fix data-race in default implementation of delayed task.

src/libplatform/default-platform.cc

Index: src/libplatform/default-platform.cc
diff --git a/src/libplatform/default-platform.cc b/src/libplatform/default-platform.cc
index 2e13cc09042a896a62aec3525890f8d72a22eb3d..b41c5852a82119aaa074e6b74b7b5c47e957df97 100644
--- a/src/libplatform/default-platform.cc
+++ b/src/libplatform/default-platform.cc
@@ -149,6 +149,7 @@ void DefaultPlatform::CallOnForegroundThread(v8::Isolate* isolate, Task* task) {
 void DefaultPlatform::CallDelayedOnForegroundThread(Isolate* isolate,
                                                     Task* task,
                                                     double delay_in_seconds) {
+  base::LockGuard<base::Mutex> guard(&lock_);
   double deadline = MonotonicallyIncreasingTime() + delay_in_seconds;
   main_thread_delayed_queue_[isolate].push(std::make_pair(deadline, task));
 }