Make AsyncHandleWaiter reenterancy-safe

ipc/mojo/async_handle_waiter.h

Index: ipc/mojo/async_handle_waiter.h
diff --git a/ipc/mojo/async_handle_waiter.h b/ipc/mojo/async_handle_waiter.h
index 55acb3eb8e2ecdec369dd56420a678b2da2be810..d6cc74510b28eb0e189faf2ae489522f8b6d63f5 100644
--- a/ipc/mojo/async_handle_waiter.h
+++ b/ipc/mojo/async_handle_waiter.h
@@ -8,6 +8,7 @@
 #include "base/callback.h"
 #include "base/memory/ref_counted.h"
 #include "base/memory/weak_ptr.h"
+#include "base/message_loop/message_loop.h"
 #include "ipc/ipc_export.h"
 #include "third_party/mojo/src/mojo/public/c/system/types.h"
 
@@ -32,6 +33,9 @@ class IPC_MOJO_EXPORT AsyncHandleWaiter {
 
   MojoResult Wait(MojoHandle handle, MojoHandleSignals signals);
 
+  base::MessageLoopForIO::IOObserver* GetIOObserverForTest();
+  base::Callback<void(MojoResult)> GetWaitCallbackForTest();
+
  private:
   void InvokeCallback(MojoResult result);