[Clank Nav Cap] Fix fullscreen WM auxiliary navigations from PWA CUJ

A new auxiliary browsing context navigation (an anchor link with
`target="_blank" rel="opener"` specified or JS code `window.open("...",
"_blank", "opener")`) should cause a new Page to be opened and navigated
to while keeping the source Page of the navigation unaffected.

Currently, on desktop windowing Clank, when this type of navigation
starts in a PWA, a new browser window is opened for the new page.

In desktop windowing mode Clank this could either:
- open a new tab in the current PWA if the url is in its scope or there
is no specialized app for the url (optioanlly showing an out of scope
banner)
- open another app (either a PWA or a native app) when the link is
in its scope.

This second case breaks the Javascript implementation for `window.open`
[1].

With this CL we want to align both cases in fullscreen windowing Clank
by always opening a tab in the current PWA window and address the bug.

While desktop windowing Clank always opens a new browser tab (and
eventually a new PWA instance, more context at [2]), for fullscreen
windowing it makes more sense to keep the navigation in the current PWA
rather than jumping to other apps or the browser.

This change is behind a flag AUXILIARY_NAVIGATION_STAYS_IN_PWA and a new
field trials experiment has been added to gradually rollout this change
and another similar case (AUXILIARY_NAVIGATION_STAYS_IN_BROWSER)
targeting fullscreen windowing Clank navigation CUJs.

A test is also added to cover this new change. git footer
Validate-Test-Flakiness: skip added to account for flaky test results
derived from external factors. Check out Patch 4 Checks for more.

Validate-Test-Flakiness: skip
Bug: 427125873
Change-Id: I4c2b29e995c81444a245af90d63359a7cf9c5620
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6799380
Reviewed-by: Michael Thiessen <mthiesse@chromium.org>
Commit-Queue: Fabio Muratori <muratori@google.com>
Cr-Commit-Position: refs/heads/main@{#1494107}