Is there a use case for this feature? I'm curious what motivated the issue.

One of possible scenarios is to run tests on try-bot to figure out whether tests are flaky or not. For this case a user can set "retries"='0' 
and if test fails, no retries will happen and execution time will decrease.
Right now "retries" is hardcoded and the value cannot be updated by config, can change only by change in ../ios/build/bots/scripts/run.py

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/tools/build/+/0d9ee2afadf2db24748fa11f5e1688cd8582371f

commit 0d9ee2afadf2db24748fa11f5e1688cd8582371f
Author: git config --global <monufriienko@chromium.org>
Date: Fri Dec 14 21:45:07 2018

Add ability to set "retries" value in config file
and use 3 as a default value (for backward compatibility
with existing tests that already use retries=3).

Recipe-Nontrivial-Roll: build_limited_scripts_slave

Bug:914984
Change-Id: I8917d8532921be9997cfc7ff4c84c9b0c4ce2d3d
Reviewed-on: https://chromium-review.googlesource.com/c/1377175
Commit-Queue: Maksym Onufriienko <monufriienko@chromium.org>
Reviewed-by: Justin Cohen <justincohen@chromium.org>
Reviewed-by: Sergey Berezin <sergeyberezin@chromium.org>

[modify] https://crrev.com/0d9ee2afadf2db24748fa11f5e1688cd8582371f/scripts/slave/recipe_modules/ios/examples/full.expected/xparallel_run_skip.json
[modify] https://crrev.com/0d9ee2afadf2db24748fa11f5e1688cd8582371f/scripts/slave/recipe_modules/ios/api.py
[modify] https://crrev.com/0d9ee2afadf2db24748fa11f5e1688cd8582371f/scripts/slave/recipe_modules/ios/examples/full.py