Lacros: Set is_chromeos_device for fishfood lacros.

crrev.com/c/2367797 removed use_cras=false, but it was not enough
to enable cras. In Chrome OS, CRAS is enabled only on build for
devices, i.e., linux-chromeos does not link against libcras.
Lacros starts to follow it.

Bug: 1118241
Change-Id: Ib7b788ba3a634620f79d1fde133332981a8d0749
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2368800Reviewed-by: Yuke Liao <liaoyuke@chromium.org>
Commit-Queue: Hidehiko Abe <hidehiko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#800910}

tools/mb/mb_config.pyl

@@ -2415,7 +2415,7 @@
     },
 
     'amd64-lacros': {
-      'gn_args': 'use_ozone=true ozone_platform_wayland=true ozone_platform_x11=false target_os="linux" use_evdev_gestures=false use_vaapi=false use_gtk=false use_glib=false enable_linux_installer=false rtc_use_pipewire=false use_gio=false use_v8_context_snapshot=false use_custom_libcxx=false use_pulseaudio=false use_pangocairo=false chromeos_is_browser_only=true use_system_libsync=false cros_host_sysroot="//build/linux/debian_sid_amd64-sysroot" cros_v8_snapshot_sysroot="//build/linux/debian_sid_amd64-sysroot" use_custom_libcxx_for_host=true'
+      'gn_args': 'use_ozone=true ozone_platform_wayland=true ozone_platform_x11=false target_os="linux" is_chromeos_device=true use_evdev_gestures=false use_vaapi=false use_gtk=false use_glib=false enable_linux_installer=false rtc_use_pipewire=false use_gio=false use_v8_context_snapshot=false use_custom_libcxx=false use_pulseaudio=false use_pangocairo=false chromeos_is_browser_only=true use_system_libsync=false cros_host_sysroot="//build/linux/debian_sid_amd64-sysroot" cros_v8_snapshot_sysroot="//build/linux/debian_sid_amd64-sysroot" use_custom_libcxx_for_host=true'
     },
 
     # We build Android with codecs on most bots to ensure maximum test