Revert "Remove shipped kill switches for External Navigation"

This reverts commit c4c44c34de9447d2698673b5731dca09b99482cb.

Reason for revert: Need to land a kill switch which requires the feature to be flagged off:
Bug: 1467532

Original change's description:
> Remove shipped kill switches for External Navigation
>
> These kill switches have shipped for two full milestones and should be
> safe to remove.
>
> Change-Id: I4385d9534a9ae2b264d61b776dc803835f115889
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4690205
> Reviewed-by: Yaron Friedman <yfriedman@chromium.org>
> Commit-Queue: Michael Thiessen <mthiesse@chromium.org>
> Code-Coverage: Findit <findit-for-me@appspot.gserviceaccount.com>
> Cr-Commit-Position: refs/heads/main@{#1171720}


Revert "Allow CCT/TWAs to bypass gesture requirement when returning to client"

This reverts commit f7fe1b9a0e644df17a5981e61e7f548de29e159b.

Reason for revert: Conflict with crbug/1467532 - easy to bring back after

Original change's description:
> Allow CCT/TWAs to bypass gesture requirement when returning to client
>
> Custom Tabs and TWAs are often used for things like sign-on, where the
> CCT/TWA is opened, then redirects back to the client. We fairly
> recently added a 15 second timeout on navigation chains so that users
> wouldn't be surprised by inactive tabs redirecting to apps. However,
> this lead to flakiness in some CCT/TWA users as in some enterprise
> scenarios login redirect chains can frequently take more than 15
> seconds to complete.
>
> Given the user is redirecting back to the app that opened the CCT/TWA
> the abuse vector is very limited (requires the user to use a malicious
> app), the user gesture/15 second timeout isn't very strong protection
> to begin with, and the user probably expects to be returned to the app
> that opened the tab, I think we can lift the restriction for this case.
>
> Bug: 1464549
> Change-Id: I2007f5dd3b2aa1f1b7be2065b9885a557e61bb40
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4696514
> Commit-Queue: Michael Thiessen <mthiesse@chromium.org>
> Reviewed-by: Yaron Friedman <yfriedman@chromium.org>
> Reviewed-by: Robert Sesek <rsesek@chromium.org>
> Cr-Commit-Position: refs/heads/main@{#1172516}

(cherry picked from commit f5c6b56c4268ef71a357eecd0558e5d5993c63ec)

Bug: 1464549
Change-Id: If9f23853b98fb3b7012b490ae980e09a8de300d6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4716039
Reviewed-by: Fred Mello <fredmello@chromium.org>
Auto-Submit: Yaron Friedman <yfriedman@chromium.org>
Commit-Queue: Yaron Friedman <yfriedman@chromium.org>
Commit-Queue: Fred Mello <fredmello@chromium.org>
Cr-Original-Commit-Position: refs/heads/main@{#1174907}
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4716846
Reviewed-by: Krishna Govind <govind@chromium.org>
Owners-Override: Krishna Govind <govind@chromium.org>
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Cr-Commit-Position: refs/branch-heads/5910@{#4}
Cr-Branched-From: 15108d301ec7fc2f21b5304bb023e8c73b1d4a88-refs/heads/main@{#1174744}