Fix pre-fetched CCTs that redirect to apps

https://chromium-review.googlesource.com/c/chromium/src/+/3780380 broke
pre-fetched (mayLaunchUrl API) CCTs that redirect to apps upon getting
shown to the user.

An example of this is login redirects that go through the browser - if
the user is already signed in on the web, then the site will redirect
when the CCT gets shown. However, when the CCT was preloaded Chrome
erroneously treats this as a browser-initiated navigation, so the
change to block browser-initiated navigations from leaving Chrome
mistakenly blocked this as well.

I also had to fix a bug around CCT closing very early on during
startup. Chrome shows the Activity Tab when the Activity is shown, but
with pre-fetched CCTs the tab may not yet be attached to a tab model
(and also not actually visible yet), so when the page redirects to an
app upon being shown we try to close the tab and end up crashing as
the tab isn't connected to a TabModel yet.

I put the change to CCT showing behind a kill-switch just in case
things unexpectedly break.

Bug: 1383509
Change-Id: I780c99cf812c2566a31681e65aa678994279bfcb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4023887
Reviewed-by: Yaron Friedman <yfriedman@chromium.org>
Reviewed-by: Ella Ge <eirage@chromium.org>
Commit-Queue: Michael Thiessen <mthiesse@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1071270}