Only observe the original active web contents when view is created.

Cq-Include-Trybots: luci.chrome.try:linux-chromeos-chrome
Bug: b:273785262
Change-Id: I301b24817faaef9b5b5f12e2e4beadeb26824a9a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4350424


Commit-Queue: Juan Mojica <juanmojica@google.com>
Commit-Queue: Ali Stanfield <stanfield@google.com>
Reviewed-by: Ali Stanfield <stanfield@google.com>
Auto-Submit: Juan Mojica <juanmojica@google.com>
Cr-Commit-Position: refs/heads/main@{#1118941}