linux/ozone: Fix gn args and add ozone_platform_headless

There was a wrong gn arg in the ozone/linux config, which
had been removed (ozone_platform = "X11").

Also, ozone_platform_headless has been added to be able to run
tests with headless backend.

Bug: 700309
Change-Id: I66ba98ab7cc55631a0af1ddfb8a06e38e4b7eaf0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1735463Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Maksim Sisov <msisov@igalia.com>
Cr-Commit-Position: refs/heads/master@{#684024}

tools/mb/mb_config.pyl

@@ -2267,8 +2267,8 @@
 
     'ozone_linux': {
       'gn_args': ('ozone_auto_platforms=false ozone_platform_wayland=true '
-                  'ozone_platform="x11" '
-                  'ozone_platform_x11=true ozone_platform_gbm=true '
+                  'ozone_platform_headless=true ozone_platform_x11=true '
+                  'ozone_platform_gbm=true '
                   'use_xkbcommon=true use_ozone=true'),
     },