Reland "X11 and Ozone: enable use_x11 and use_ozone"

This is a reland of f985905e.

This change broke our headless build:
ERROR at //ui/events/platform/x11/BUILD.gn:51:5: Assertion failed.
    assert(ozone_platform_x11)
    ^-----
See //components/viz/demo/BUILD.gn:73:7: which caused the file to be included.
      "//ui/events/platform/x11",.

Fix:
Don't include x11 build for headless case.

Original change's description:
> X11 and Ozone: enable use_x11 and use_ozone
>
> This is the final change that enables use_x11
> and use_ozone for Linux builds and (as it was
> before) excludes X11 for chromeos, chromecast,
> lacros.
>
> in order to run ozone/X11/wayland, one must pass
> —enable-features=UseOzonePlatform
> —ozone-platform=x11 or wayland. otherwise,
> non-ozone/X11 is used.
>
> it’s also possible to completely disable X11
> so that clients, who need that kind of build can
> continue to have that.
>
> Bug: 1085700
> Change-Id: I171bcb53c406555a1d4bec3e88d76dc676310d48
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2315869
> Reviewed-by: Thomas Anderson <thomasanderson@chromium.org>
> Reviewed-by: Robert Kroeger <rjkroege@chromium.org>
> Reviewed-by: Scott Violet <sky@chromium.org>
> Commit-Queue: Maksim Sisov (GMT+3) <msisov@igalia.com>
> Cr-Commit-Position: refs/heads/master@{#801523}

Bug: 1085700
Change-Id: I8bf0abead6c94c95ddc58443de7cf2268c977cf1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2376806Reviewed-by: Thomas Anderson <thomasanderson@chromium.org>
Commit-Queue: Maksim Sisov (GMT+3) <msisov@igalia.com>
Cr-Commit-Position: refs/heads/master@{#801692}

build/args/headless.gn

@@ -45,3 +45,6 @@ v8_enable_lazy_source_positions = false
 use_glib = false
 use_gtk = false
 use_pangocairo = false
+
+# TODO(1096425): Remove this once use_x11 goes away.
+use_x11 = false

build/config/ui.gni

@@ -18,11 +18,19 @@
 # of //build/config/BUILDCONFIG.gn.
 
 import("//build/config/chromecast_build.gni")
+import("//build/config/chromeos/ui_mode.gni")
 
 declare_args() {
   # Indicates if Ozone is enabled. Ozone is a low-level library layer for Linux
-  # that does not require X11. Enabling this feature disables use of x11.
-  use_ozone = is_chromeos || (is_chromecast && !is_android) || is_fuchsia
+  # that does not require X11.
+  use_ozone =
+      is_chromeos || (is_chromecast && !is_android) || is_fuchsia || is_linux
+
+  # Indicates if the UI toolkit depends on X11.
+  # Enabled by default. Can be disabled if Ozone only build is required and
+  # vice-versa.
+  use_x11 =
+      is_linux && !is_chromecast && !is_chromeos && !chromeos_is_browser_only
 
   # Indicates if Aura is enabled. Aura is a low-level windowing library, sort
   # of a replacement for GDI or GTK.
@@ -37,13 +45,6 @@ declare_args() {
   use_glib = is_desktop_linux && !is_chromecast
 }
 
-# Additional dependent variables -----------------------------------------------
-#
-# These variables depend on other variables and can't be set externally.
-
-# Indicates if the UI toolkit depends on X11.
-use_x11 = is_linux && !use_ozone
-
 # Make sure glib is not used if building for ChromeOS/Chromecast
 assert(!use_glib || (is_linux && !is_chromeos && !is_chromecast))