Mojo: In Debug builds, log when a race condition in user code is likely observed.

mojo/system/handle_table.cc

Index: mojo/system/handle_table.cc
diff --git a/mojo/system/handle_table.cc b/mojo/system/handle_table.cc
index 2e4d22a98244ab6517e57c69949160e4bca3e6e1..9ab2d24d20b4c9c544106568ee32240c0d9f6979 100644
--- a/mojo/system/handle_table.cc
+++ b/mojo/system/handle_table.cc
@@ -146,6 +146,13 @@ MojoResult HandleTable::MarkBusyAndStartTransport(
         Dispatcher::HandleTableAccess::TryStartTransport(
             entries[i]->dispatcher.get());
     if (!transport.is_valid()) {
+      // Only log for Debug builds, since this is not a problem with the system
+      // code, but with user code.
+      DLOG(WARNING) << "Likely race condition in user code detected: attempt "
+                       "to transfer handle "
+                    << handles[i]
+                    << " while it is in use on a different thread";
+
       // Unset the busy flag (since it won't be unset below).
       entries[i]->busy = false;
       error_result = MOJO_RESULT_BUSY;