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If the new (DISABLED_BY_DEFAULT) IgnoreEncryptionKeysLongMissing feature
is enabled, ModelTypeWorker will ignore both future and pending updates
encrypted with keys that remained unknown for 'too long', as measured by
UnknownEncryptionKeyInfo::gu_responses_while_should_have_been_known.
The threshold for 'too long' is arbitrary for the moment. Two different
metrics are added: the first is logged when a key starts being ignored,
and records the number of pending updates that were dropped. The second
is recorded individually for each incoming update entity that's
encrypted with an ignored key.

Future CLs will start persisting the notion of unknown keys across
browser restarts.

Bug: 1109221
Change-Id: I52b841368385ce347fbaa8746501ab07a5b7752a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2567883Reviewed-by: Steven Holte <holte@chromium.org>
Reviewed-by: Mikel Astiz <mastiz@chromium.org>
Commit-Queue: Victor Vianna <victorvianna@google.com>
Cr-Commit-Position: refs/heads/master@{#835108}