Prevent race condition while switching surfaces (#24838)

https://dart.googlesource.com/external/github.com/flutter/engine/+/9662a38a79732fc9090bb43a47003d8e063ee517
diff --git a/commits.json b/commits.json
index d17a72c..9a72a2c 100644
--- a/commits.json
+++ b/commits.json
@@ -1,5 +1,5 @@
 {
-  "external/github.com/flutter/engine":"ce21b840bac94c88ccc92890ac9a1b3b5318710b",
+  "external/github.com/flutter/engine":"9662a38a79732fc9090bb43a47003d8e063ee517",
   "external/github.com/flutter/flutter":"a0ba6464089f906fefbf610732b2c9a7b66a1260",
   "sdk":"c8d6f85d0f70fb870e7396fbc224817973766552"
 }
\ No newline at end of file