[bedrock] Introduce Browser::DeprecatedCreateOwnedForTesting()

This CL is purely a refactor, there are no behavioral changes.

Currently Browser::Create returns an unowned Browser pointer in
production code, with the Browser ultimately being owned by its
BrowserView / NativeWidget.

However current Browser unittests employ BrowserWindow mocks and
take direct ownership over the Browser returned by Browser::Create().

Browser::DeprecatedCreateOwnedForTesting() is added to make it clear
what the resulting ownership model for the created Browser instance
should be. Currently this is a no-op, but this distinction will
simplify the transition to the new ownership model.

Bug: 413168662
Change-Id: Idca9d7bf035dd3abecfacab6fbf68d7c648978d2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6656392
Commit-Queue: Tom Lukaszewicz <tluk@chromium.org>
Reviewed-by: Erik Chen <erikchen@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1476817}