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..d1b3582baf58751e8602abc51aafc1c921436772 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::_;
@@ -124,14 +125,45 @@ TEST_F(WebThreadImplForWorkerSchedulerTest, TestDefaultTask) {
 TEST_F(WebThreadImplForWorkerSchedulerTest,
        TestTaskExecutedBeforeThreadDeletion) {
   scoped_ptr<MockTask> task(new MockTask());
-  base::WaitableEvent completion(false, false);
+  bool task_ran = false;
+  base::Lock lock;
 
   EXPECT_CALL(*task, run());
   ON_CALL(*task, run())
-      .WillByDefault(Invoke([&completion]() { completion.Signal(); }));
+      .WillByDefault(Invoke([&task_ran, &lock]() {
+        base::AutoLock auto_lock(lock);
+        task_ran = true;
+      }));
+
+  thread_->postTask(blink::WebTraceLocation(), task.release());
+  thread_.reset();
+  {
+    base::AutoLock auto_lock(lock);
+    EXPECT_TRUE(task_ran);
+  }
+}
 
+TEST_F(WebThreadImplForWorkerSchedulerTest,
+       TestThreadShutdownAfterPreShutdown) {
+  scoped_ptr<MockTask> task(new MockTask());
+  bool task_ran = false;
+  base::Lock lock;
+
+  ON_CALL(*task, run())
+      .WillByDefault(Invoke([&task_ran, &lock]() {
+        base::AutoLock auto_lock(lock);
+        task_ran = true;
+      }));
+
+  // preShutdown should block the postTask, but not the internal shutdown task.
+  thread_->scheduler()->preShutdown();
   thread_->postTask(blink::WebTraceLocation(), task.release());
   thread_.reset();
+
+  {
+    base::AutoLock auto_lock(lock);
+    EXPECT_FALSE(task_ran);
+  }
 }
 
 TEST_F(WebThreadImplForWorkerSchedulerTest, TestIdleTask) {