ProxyChain: Ignore invalid cached proxy settings

The valid configurations of ProxyChain change between versions and build
flags and is not guaranteed to be backward compatible.
Disk cached http responses persist between builds, so we should ignore
invalid configurations, as it will trigger the CHECK in the
constructor of ProxyChain once the invalid struct is passed over mojo.

Change-Id: I8cd4ad0ef735632ab8b51bd3aacd0513a7f51f1f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6005858
Reviewed-by: Kenichi Ishibashi <bashi@chromium.org>
Reviewed-by: Adam Rice <ricea@chromium.org>
Commit-Queue: Tal Keren <tkeren@paloaltonetworks.com>
Cr-Commit-Position: refs/heads/main@{#1385558}