[mojo-edk] Expose notification source to MojoWatch callbacks

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 f72f495fd683874e325ba24976226e03508f1649..2d23cbf3a56565eebcb313f4ea88746b2eb2aec9 100644
--- a/mojo/public/cpp/system/tests/watcher_unittest.cc
+++ b/mojo/public/cpp/system/tests/watcher_unittest.cc
@@ -202,5 +202,30 @@ TEST_F(WatcherTest, CloseAndCancel) {
   base::RunLoop().RunUntilIdle();
 }
 
+TEST_F(WatcherTest, SyncDispatch) {
+  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_OK, result);
+                              notified = true;
+                            })));
+  EXPECT_TRUE(b_watcher.IsWatching());
+
+  b_watcher.EnableImmediateDispatchOfEventsFromSameThread(true);
+  EXPECT_EQ(MOJO_RESULT_OK, WriteMessageRaw(a.get(), "hello", 5, nullptr, 0,
+                                            MOJO_WRITE_MESSAGE_FLAG_NONE));
+
+  // We don't spin the MessageLoop but should have been notified anyway.
+
+  EXPECT_TRUE(notified);
+
+  b_watcher.Cancel();
+}
+
 }  // namespace
 }  // namespace mojo