+pkl for input.  Is one milestone enough of a buffer to update serialization keys?  We also have other serialization code to handle legacy serialization keys; when do you think it would be appropriate to remove them?

Let's talk in person about the details.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/6de946db9c3290cf74a61595962116b6d5384e45

commit 6de946db9c3290cf74a61595962116b6d5384e45
Author: Sylvain Defresne <sdefresne@chromium.org>
Date: Thu Aug 02 13:52:03 2018

Remove support for legacy WebState user data format

The format was changed in M-57, so legacy code can now be dropped
as M-69 has been branched.

Bug: 661633,  691800 
Cq-Include-Trybots: luci.chromium.try:ios-simulator-full-configs;master.tryserver.chromium.mac:ios-simulator-cronet
Change-Id: I8cd160acfb994091658939662534d578881c981c
Reviewed-on: https://chromium-review.googlesource.com/1158698
Commit-Queue: Sylvain Defresne <sdefresne@chromium.org>
Reviewed-by: Eugene But <eugenebut@chromium.org>
Cr-Commit-Position: refs/heads/master@{#580155}
[modify] https://crrev.com/6de946db9c3290cf74a61595962116b6d5384e45/ios/web/BUILD.gn
[modify] https://crrev.com/6de946db9c3290cf74a61595962116b6d5384e45/ios/web/navigation/serializable_user_data_manager_impl.h
[modify] https://crrev.com/6de946db9c3290cf74a61595962116b6d5384e45/ios/web/navigation/serializable_user_data_manager_impl.mm
[delete] https://crrev.com/4fc490dcb251520ce0c3661a4e3fad4eb2d3429b/ios/web/navigation/serializable_user_data_manager_impl_unittest.mm