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Currently, the keyboard accessory may show stale suggestions. This is
because sheets are cached by adding them with `emplace` to a flat_map.
`emplace` only adds an item if it isn't present, so the oldest,
non-empty sheet (for a given tab) will be shown forever. This isn't
immediately obvious since sheets are also pushed directly when they are
updated but with indirect updates (e.g. updating another sheet), the
stale value will override the new one.

By using `insert_or_assign`, the newer value replaces the old one.

Bug: 1140358, 1134119
Change-Id: I933ec47a1da596f58fa99d5b9bc481954122c2a3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2489899Reviewed-by: Vasilii Sukhanov <vasilii@chromium.org>
Commit-Queue: Friedrich [CET] <fhorschig@chromium.org>
Cr-Commit-Position: refs/heads/master@{#820165}