Initialize next_pointer_flow_id_ to 0 (#9026)

This member variable was added to three classes in #7807, but only
initialized to 0 in one of them. Initialize to 0 in the other two.
https://dart.googlesource.com/external/github.com/flutter/engine/+/e40768c63a00252062ea1061a6db9e756ccb65e8
diff --git a/commits.json b/commits.json
index f104079..4c81020 100644
--- a/commits.json
+++ b/commits.json
@@ -1,5 +1,5 @@
 {
-  "external/github.com/flutter/engine":"3d2e9b24aa712ba60206e7ac392a709df0939635",
+  "external/github.com/flutter/engine":"e40768c63a00252062ea1061a6db9e756ccb65e8",
   "external/github.com/flutter/flutter":"d0c603d0bbf8898038f620fe47c3552ece07fd65",
   "sdk":"2a85d595ba5541c01d3fd8961c929f9ac4695597"
 }
\ No newline at end of file