Set ozone_platform args for Chrome OS Lacros build

This CL changes the parameters around ozone platform when
|is_chromeos_lacros| is true.
This is a part of Lacros args.gn simplification (go/simplelacros).

Previously, the ozone platform parameters are set in the same way to
is_linux build.
After this CL, |ozone_platform| and |ozone_platform_x11| value is changed
for is_chromeos_lacros build.

Bug: 1203201
Change-Id: I701e4a08617ef7c4aec5108a46077ab4bd09bb02
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2902958
Reviewed-by: Hidehiko Abe <hidehiko@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Commit-Queue: Eriko Kurimoto <elkurin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#887825}