[ash+lacros] Guard preloading `nacl_helper` with ENABLE_NACL

In ash/chrome DUT builds with `enable_nacl = false` in GN args,
ash/chrome fails to launch because `nacl_helper` is attempted to
preload unconditionally.

The failure happens in PreloadFile() function with

>  FATAL chrome[21630:21647]: [browser_manager.cc(197)]
>  	Check failed: file.IsValid(). : No such file or directory (2)

... where the file path is `/usr/local/lacros-chrome/nacl_helper`.

BUG=None
R=erikchen@chromium.org

Change-Id: I9e7745ada633c591232343828cb1856c646d915c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4638910


Auto-Submit: Antonio Gomes <tonikitoo@igalia.com>
Reviewed-by: Erik Chen <erikchen@chromium.org>
Commit-Queue: Erik Chen <erikchen@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1161971}