Mojo: Don't hold a watch context ref in SimpleWatcher posted task

mojo/public/cpp/system/simple_watcher.h

Index: mojo/public/cpp/system/simple_watcher.h
diff --git a/mojo/public/cpp/system/simple_watcher.h b/mojo/public/cpp/system/simple_watcher.h
index 5e5a7ea3a101a24eb8b1b293f32cf8554cc0cf0c..9001884c97360658648098eadde93057d88aab25 100644
--- a/mojo/public/cpp/system/simple_watcher.h
+++ b/mojo/public/cpp/system/simple_watcher.h
@@ -164,7 +164,7 @@ class MOJO_CPP_SYSTEM_EXPORT SimpleWatcher {
  private:
   class Context;
 
-  void OnHandleReady(scoped_refptr<const Context> context, MojoResult result);
+  void OnHandleReady(int watch_id, MojoResult result);
 
   base::ThreadChecker thread_checker_;
 
@@ -190,6 +190,10 @@ class MOJO_CPP_SYSTEM_EXPORT SimpleWatcher {
   // The handle currently under watch. Not owned.
   Handle handle_;
 
+  // A simple counter to disambiguate notifications from multiple watch contexts
+  // in the event that this SimpleWatcher cancels and watches multiple times.
+  int watch_id_ = 0;
+
   // The callback to call when the handle is signaled.
   ReadyCallback callback_;