[Clank Nav Cap] Top-level navigation from PWA to itself CUJ update

This CL implements a new navigation CUJ for Clank.

New behavior:

A new top level browsing context navigation (target="_blank" with no
additional modifier) starting in PWA task named A and towards a link in
scope of PWA A itself, should always cause a new task to be created even
if existing ones are available.

Background:

This is a specific behavior implemented in Desktop Chrome, where the
Launch Handler API is supported. This API provides customizable opening
logic for PWAs using the client_mode attribute.

Clank should already be supporting Launch Handler for Trusted Web
Activities from v138 (behind flag).

client_mode (a web app manifest.json attribute) defines 3 possible
policies for PWA opening:
- focus_existing: if a task is already open, focus it and do not navigate
- navigate_existing: if a task is already open, focus and navigate
- navigate_new: always open a new task, regardless of pre-existing ones

For PWA to itself navigations in new top-level browsing context
specifically, an edge case is applied on top of the API specification.
Instead of following the client_mode logic, a new task should be opened
every time.

Bug: 382776297
Change-Id: Ie430e17323877c1d4b3d5871ebc7620319219206
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6519655
Reviewed-by: Michael Thiessen <mthiesse@chromium.org>
Commit-Queue: Fabio Muratori <muratori@google.com>
Cr-Commit-Position: refs/heads/main@{#1506188}