The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/5daaf4ad01fcca266fdbd3024408d02024bc343f

commit 5daaf4ad01fcca266fdbd3024408d02024bc343f
Author: Sebastien Lalancette <seblalancette@chromium.org>
Date: Fri Jan 11 22:09:25 2019

[AF] Added Autocomplete Retention Policy feature flag to chrome://flags.

The entry will be removed as part of the overall flag cleanup:
crbug.com/920214

Also fixed a typo in the feature flag name.

Bug:  920741 
Change-Id: Icefacf04b521d2c5195449c57ce3d310631bd310
Reviewed-on: https://chromium-review.googlesource.com/c/1406933
Commit-Queue: Sebastien Lalancette <seblalancette@chromium.org>
Reviewed-by: Fabio Tirelo <ftirelo@chromium.org>
Reviewed-by: Elly Fong-Jones <ellyjones@chromium.org>
Cr-Commit-Position: refs/heads/master@{#622179}
[modify] https://crrev.com/5daaf4ad01fcca266fdbd3024408d02024bc343f/chrome/browser/about_flags.cc
[modify] https://crrev.com/5daaf4ad01fcca266fdbd3024408d02024bc343f/chrome/browser/flag-metadata.json
[modify] https://crrev.com/5daaf4ad01fcca266fdbd3024408d02024bc343f/chrome/browser/flag_descriptions.cc
[modify] https://crrev.com/5daaf4ad01fcca266fdbd3024408d02024bc343f/chrome/browser/flag_descriptions.h
[modify] https://crrev.com/5daaf4ad01fcca266fdbd3024408d02024bc343f/components/autofill/core/browser/autocomplete_history_manager.cc
[modify] https://crrev.com/5daaf4ad01fcca266fdbd3024408d02024bc343f/components/autofill/core/browser/autocomplete_history_manager_unittest.cc
[modify] https://crrev.com/5daaf4ad01fcca266fdbd3024408d02024bc343f/components/autofill/core/browser/webdata/autocomplete_sync_bridge.cc
[modify] https://crrev.com/5daaf4ad01fcca266fdbd3024408d02024bc343f/components/autofill/core/browser/webdata/autocomplete_sync_bridge_unittest.cc
[modify] https://crrev.com/5daaf4ad01fcca266fdbd3024408d02024bc343f/components/autofill/core/browser/webdata/autofill_table.cc
[modify] https://crrev.com/5daaf4ad01fcca266fdbd3024408d02024bc343f/components/autofill/core/browser/webdata/autofill_table_unittest.cc
[modify] https://crrev.com/5daaf4ad01fcca266fdbd3024408d02024bc343f/components/autofill/core/common/autofill_features.cc
[modify] https://crrev.com/5daaf4ad01fcca266fdbd3024408d02024bc343f/components/autofill/core/common/autofill_features.h
[modify] https://crrev.com/5daaf4ad01fcca266fdbd3024408d02024bc343f/ios/chrome/browser/about_flags.mm
[modify] https://crrev.com/5daaf4ad01fcca266fdbd3024408d02024bc343f/ios/chrome/browser/ios_chrome_flag_descriptions.cc
[modify] https://crrev.com/5daaf4ad01fcca266fdbd3024408d02024bc343f/ios/chrome/browser/ios_chrome_flag_descriptions.h
[modify] https://crrev.com/5daaf4ad01fcca266fdbd3024408d02024bc343f/tools/metrics/histograms/enums.xml

Should be able to see "Enable automatic cleanup of expired Autocomplete entries" in chrome://flags on all platforms.

Issue verified 
Version: Chrome Canary 73.0.3672.0
Device: iPhone 6S
iOS: 12.1.2

Able to see "Enable automatic cleanup of expired Autocomplete entries" in chrome://flags
https://drive.google.com/open?id=1Qkewe_fJRyEtlV4zIU52dZvwdZuALEBN