Revert of Remove MessageLoop destruction observer from mojo::Watcher.

mojo/public/cpp/system/tests/watcher_unittest.cc

Index: mojo/public/cpp/system/tests/watcher_unittest.cc
diff --git a/mojo/public/cpp/system/tests/watcher_unittest.cc b/mojo/public/cpp/system/tests/watcher_unittest.cc
index ebeebf6777888a67aeeba7af1b6bf0290cd2d312..5cb3ed23b7799571da5820a1d81492791b8267f1 100644
--- a/mojo/public/cpp/system/tests/watcher_unittest.cc
+++ b/mojo/public/cpp/system/tests/watcher_unittest.cc
@@ -8,7 +8,6 @@
 
 #include "base/bind.h"
 #include "base/callback.h"
-#include "base/macros.h"
 #include "base/message_loop/message_loop.h"
 #include "base/run_loop.h"
 #include "base/threading/thread_task_runner_handle.h"
@@ -36,10 +35,16 @@
   WatcherTest() {}
   ~WatcherTest() override {}
 
- private:
-  base::MessageLoop message_loop_;
-
-  DISALLOW_COPY_AND_ASSIGN(WatcherTest);
+  void SetUp() override {
+    message_loop_.reset(new base::MessageLoop);
+  }
+
+  void TearDown() override {
+    message_loop_.reset();
+  }
+
+ protected:
+  std::unique_ptr<base::MessageLoop> message_loop_;
 };
 
 TEST_F(WatcherTest, WatchBasic) {
@@ -157,6 +162,28 @@
   run_loop.Run();
 }
 
+TEST_F(WatcherTest, NotifyOnMessageLoopDestruction) {
+  ScopedMessagePipeHandle a, b;
+  CreateMessagePipe(nullptr, &a, &b);
+
+  bool notified = false;
+  Watcher b_watcher;
+  EXPECT_EQ(MOJO_RESULT_OK,
+            b_watcher.Start(b.get(), MOJO_HANDLE_SIGNAL_READABLE,
+                            OnReady([&] (MojoResult result) {
+                              EXPECT_EQ(MOJO_RESULT_ABORTED, result);
+                              notified = true;
+                            })));
+  EXPECT_TRUE(b_watcher.IsWatching());
+
+  message_loop_.reset();
+
+  EXPECT_TRUE(notified);
+
+  EXPECT_TRUE(b_watcher.IsWatching());
+  b_watcher.Cancel();
+}
+
 TEST_F(WatcherTest, CloseAndCancel) {
   ScopedMessagePipeHandle a, b;
   CreateMessagePipe(nullptr, &a, &b);