[ChromeOS] Use extension helper to derive CWS URL for OnTask

This change leverages an extension helper in the OnTask navigation
throttle to derive the CWS URL over the hardcoded value. This reduces
the risk of potential break should the CWS URL change in the future.

Bug: b:404640914
Change-Id: I08a2d643361bd403a2e168dd39774095f0f09b3f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6378487
Reviewed-by: Luke Cao <caott@google.com>
Commit-Queue: Vignesh Shenvi <vshenvi@google.com>
Cr-Commit-Position: refs/heads/main@{#1435746}