[web test] Use devtools-frontend for chrome branded builds

With https://crrev.com/c/6031797 we ship devtools-frontend again for
Chrome branded builds. This CL updates the web test runner to also
use the correct DevTools build output to host the frontend.

Bug: 379040628
Change-Id: I6523adf479ad0c781e16ede4f6819003c5d4f50d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6070166
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Mariam Ali <alimariam@google.com>
Owners-Override: Mariam Ali <alimariam@google.com>
Cr-Commit-Position: refs/heads/main@{#1391583}