Roll Fuchsia Mac SDK from Xkten1Nxc... to Ler-DhxPF... (#24849)

https://dart.googlesource.com/external/github.com/flutter/engine/+/55bb5c909d91af32ef2d2ce439e13a167be4cd73
diff --git a/commits.json b/commits.json
index 17957a6..1121d10 100644
--- a/commits.json
+++ b/commits.json
@@ -1,5 +1,5 @@
 {
-  "external/github.com/flutter/engine":"ac795b83bbe2e1e309d3db342385e7ad73967acc",
+  "external/github.com/flutter/engine":"55bb5c909d91af32ef2d2ce439e13a167be4cd73",
   "external/github.com/flutter/flutter":"a0ba6464089f906fefbf610732b2c9a7b66a1260",
   "sdk":"8f2ca0b2482bf6acde1e90aab3745de407d4e51a"
 }
\ No newline at end of file