Reset requires_post_process_selection state when we process the selection.

This was causing an issue where we'd keep re-setting the same selection if we kept receiving accessibility events unrelated to selection.

Fixed: 1446993
Change-Id: I135721ce693d4305a026eec259d3b91e973ace0d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4548375


Commit-Queue: Kristi Saney <kristislee@google.com>
Reviewed-by: Abigail Klein <abigailbklein@google.com>
Cr-Commit-Position: refs/heads/main@{#1147324}