Fix flaky update content tests

The selection nodes were null when trying to forward the selection to
the main page, which is not being tested in these tests, so we can just
skip that step for these tests.

Bug: 329162803
Change-Id: I5b1caf200e2c4c6619d5a0279c259bcdfac359b8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5406824


Auto-Submit: Kristi Saney <kristislee@google.com>
Reviewed-by: Lauren Winston <lwinston@google.com>
Commit-Queue: Lauren Winston <lwinston@google.com>
Cr-Commit-Position: refs/heads/main@{#1280484}