Adds a SHUTDOWN_TASK_QUEUE and a PreShutdown api to the scheduler.

components/scheduler/child/webthread_impl_for_worker_scheduler_unittest.cc

Index: components/scheduler/child/webthread_impl_for_worker_scheduler_unittest.cc
diff --git a/components/scheduler/child/webthread_impl_for_worker_scheduler_unittest.cc b/components/scheduler/child/webthread_impl_for_worker_scheduler_unittest.cc
index 7bb1f8990762e426df7004d2c12637105c542941..52c036160c15823de117f63d29e25fc3a1c5b0ec 100644
--- a/components/scheduler/child/webthread_impl_for_worker_scheduler_unittest.cc
+++ b/components/scheduler/child/webthread_impl_for_worker_scheduler_unittest.cc
@@ -8,6 +8,7 @@
 #include "components/scheduler/child/worker_scheduler_impl.h"
 #include "testing/gmock/include/gmock/gmock.h"
 #include "testing/gtest/include/gtest/gtest.h"
+#include "third_party/WebKit/public/platform/WebScheduler.h"
 #include "third_party/WebKit/public/platform/WebTraceLocation.h"
 
 using testing::_;
@@ -132,6 +133,23 @@ TEST_F(WebThreadImplForWorkerSchedulerTest,
 
   thread_->postTask(blink::WebTraceLocation(), task.release());
   thread_.reset();
+  completion.Wait();
+}
+
+TEST_F(WebThreadImplForWorkerSchedulerTest,
+       TestThreadShutdownAfterPreShutdown) {
+  scoped_ptr<MockTask> task(new MockTask());
+  base::WaitableEvent completion(false, false);
+
+  ON_CALL(*task, run())
+      .WillByDefault(Invoke([&completion]() { completion.Signal(); }));
+
+  // preShutdown should block the postTask, but not the internal shutdown task.
+  thread_->scheduler()->preShutdown();
+  thread_->postTask(blink::WebTraceLocation(), task.release());
+  thread_.reset();
+
+  EXPECT_FALSE(completion.IsSignaled());
 }
 
 TEST_F(WebThreadImplForWorkerSchedulerTest, TestIdleTask) {