Canonicalize selection after editing commands execution.

This is a prerequisite for us to remove visible selection in DOM
selection code. Each command should put selection in a canonicalized
position after execution.

Bug: 41311101
Change-Id: I5086a3bf49c72106c09fce61093f248c13c86662
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5399116


Reviewed-by: Koji Ishii <kojii@chromium.org>
Commit-Queue: Siye Liu <siliu@microsoft.com>
Cr-Commit-Position: refs/heads/main@{#1282117}