mark complex_layout_scroll_perf_ios__timeline_summary as flaky\ (#14080)

dev/devicelab/manifest.yaml

diff --git a/dev/devicelab/manifest.yaml b/dev/devicelab/manifest.yaml
index 7f204d1..566dd61 100644
--- a/dev/devicelab/manifest.yaml
+++ b/dev/devicelab/manifest.yaml
@@ -276,6 +276,7 @@
       iOS.
     stage: devicelab_ios
     required_agent_capabilities: ["mac/ios"]
+    flaky: true
 
   flutter_gallery_ios__start_up:
     description: >