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Previously, cached values in the TouchSelectionController would be reset
to force future selection updates. However, these cached values can
actually be used outside of selection update calls, e.g., when force
showing the selection from the current cached values. Instead of
resetting the cached values, simply set a dirty bit that forces an
update, avoiding issues when dealing with the reset values.

BUG=393025

Review URL: https://codereview.chromium.org/1127383007

Cr-Commit-Position: refs/heads/master@{#329325}