[Editing] Fire selectionchange event for backspace input

The selectionchange event does not get fired when the backspace key
deletes text inside a text node. According to the specification, when
the selection is associated or dissociated with its range, the user
agent must schedule a selectionchange event on the document. This
change ensures that the event will be scheduled as per the
specification.

https://w3c.github.io/selection-api/#selectionchange-event

Bug: 41321247
Change-Id: I6c3067652a47b4c686c47f9b181261ec19888e78
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5948203
Reviewed-by: Siye Liu <siliu@microsoft.com>
Reviewed-by: Kent Tamura <tkent@chromium.org>
Commit-Queue: Tanu Jain <tanujain@microsoft.com>
Cr-Commit-Position: refs/heads/main@{#1378113}