Simplify ChromeOS guards in management ui handler

The IS_CHROMEOS guard consists of IS_CHROMEOS_LACROS and
IS_CHROMEOS_ASH. In the management ui handler there are several
guards with BUILDFLAG(IS_CHROMEOS_ASH) ||
BUILDFLAG(IS_CHROMEOS_LACROS), which can be simplified to
BUILDFLAG(IS_CHROMEOS). This drive-by fix changes that.

Change-Id: I676fd9050d676bf0c7ba25b5fdde44d15cb156a7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5424640


Reviewed-by: Denis Kuznetsov <antrim@chromium.org>
Commit-Queue: Denis Kuznetsov <antrim@chromium.org>
Auto-Submit: Simon Hangl <simonha@google.com>
Cr-Commit-Position: refs/heads/main@{#1283301}