[Devicelab] Mark microbenchmarks_ios as flaky. (#49639)

diff --git a/dev/devicelab/manifest.yaml b/dev/devicelab/manifest.yaml
index 7bb7cd2..babbb57 100644
--- a/dev/devicelab/manifest.yaml
+++ b/dev/devicelab/manifest.yaml
@@ -508,6 +508,8 @@
     stage: devicelab_ios
     required_agent_capabilities: ["mac/ios"]
     timeout_in_minutes: 30
+    # TODO(wutong): Timeout on iOS 13. https://github.com/flutter/flutter/issues/49635
+    flaky: true
 
   flutter_view_ios__start_up:
     description: >