Fix handle_watcher when start is called twice on the same handle.

mojo/common/handle_watcher_unittest.cc

Index: mojo/common/handle_watcher_unittest.cc
diff --git a/mojo/common/handle_watcher_unittest.cc b/mojo/common/handle_watcher_unittest.cc
index 3e879d28868b4e89cf3b5421a49a2544a169d30b..46d2a27c56232453b3a035ed6b122df0590f38cc 100644
--- a/mojo/common/handle_watcher_unittest.cc
+++ b/mojo/common/handle_watcher_unittest.cc
@@ -280,6 +280,22 @@ TEST_P(HandleWatcherTest, Restart) {
   EXPECT_FALSE(callback_helper2.got_callback());
 }
 
+// Verifies Start() invoked a second time on the same handle works.
+TEST_P(HandleWatcherTest, RestartOnSameHandle) {
+  MessagePipe test_pipe;
+  CallbackHelper callback_helper;
+  ASSERT_TRUE(test_pipe.handle0.is_valid());
+
+  HandleWatcher watcher;
+  callback_helper.Start(&watcher, test_pipe.handle0.get());
+  RunUntilIdle();
+  EXPECT_FALSE(callback_helper.got_callback());
+
+  callback_helper.Start(&watcher, test_pipe.handle0.get());
+  RunUntilIdle();
+  EXPECT_FALSE(callback_helper.got_callback());
+}
+
 // Verifies deadline is honored.
 TEST_P(HandleWatcherTest, Deadline) {
   InstallTickClock();