Use non-dummy LASTCHANGE for all Cast builds.

A dummy last change of @000... is generated for non-official chrome
builds, which is then used to generate the link to Cast devtools. The
resulting link
https://chrome-devtools-frontend.appspot.com/serve_rev/@0000000000000000000000000000000000000000/inspector.html
always returns 404, thus the Cast devtools window fails to load. This
change forces all Cast builds to generate proper LASTCHANGE per current
Git revision, thus enabling devtools for all Cast builds.

Fixed: b:376890796
Change-Id: I76df160916e7ae19ac15caeecb7efd543c308d92
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6094073
Reviewed-by: Simeon Anfinrud <sanfin@chromium.org>
Commit-Queue: Vigen Issahhanjan <vigeni@google.com>
Cr-Commit-Position: refs/heads/main@{#1396281}