Make GlobalDescriptors::MaybeGet return -1 when the key is not found.

media/audio/cross_process_notification_unittest.cc

Index: media/audio/cross_process_notification_unittest.cc
diff --git a/media/audio/cross_process_notification_unittest.cc b/media/audio/cross_process_notification_unittest.cc
index a5ff7b0f2df712faa89b4ee5f5b2086b5ab61417..0d3242f5055647652a9b9e5f9e45399efb595dea 100644
--- a/media/audio/cross_process_notification_unittest.cc
+++ b/media/audio/cross_process_notification_unittest.cc
@@ -394,7 +394,7 @@ struct CrudeIpc {
 // The main routine of the child process.  Waits for the parent process
 // to copy handles over to the child and then uses a CrossProcessNotification to
 // wait and signal to the parent process.
-MULTIPROCESS_TEST_MAIN(CrossProcessNotificationChildMain) {
+MULTIPROCESS_TEST_MAIN(CrossProcessNotificationChildMain, NULL) {
   base::SharedMemory mem;
   bool ok = mem.CreateNamed(
       CrossProcessNotificationMultiProcessTest::kSharedMemName,