Change default Ozone platform to Ozone X11.

ui/ozone/ozone.gni

Index: ui/ozone/ozone.gni
diff --git a/ui/ozone/ozone.gni b/ui/ozone/ozone.gni
index b677b48cf98e051a48a51eef150b85569671bccb..a330ec1b20ad15880e86d3ee6234f0b62db98852 100644
--- a/ui/ozone/ozone.gni
+++ b/ui/ozone/ozone.gni
@@ -18,18 +18,27 @@ declare_args() {
 }
 
 declare_args() {
-  # The platform that will be active by default.
+  # The platform that will used at runtime by default. This can be overridden
+  # with the command line flag --ozone-platform=<platform>.
   ozone_platform = ""
 
-  # Enable individual platforms.
+  # Compile the 'cast' platform.
   ozone_platform_cast = false
+
+  # Compile the 'gbm' platform.
   ozone_platform_gbm = false
+
+  # Compile the 'headless' platform.
   ozone_platform_headless = false
+
+  # Compile the 'x11' platform.
   ozone_platform_x11 = false
+
+  # Compile the 'wayland' platform.
   ozone_platform_wayland = false
 
   if (ozone_auto_platforms) {
-    # Use headless as the default platform.
+    # Use headless as the default platform unless modified below.
     ozone_platform = "headless"
     ozone_platform_headless = true
 
@@ -49,10 +58,12 @@ declare_args() {
         ozone_platform = "cast"
       }
     } else if (is_chromeos) {
+      ozone_platform = "x11"
       ozone_platform_gbm = true
       ozone_platform_x11 = true
       ozone_platform_wayland = true
     } else if (is_desktop_linux) {
+      ozone_platform = "x11"
       ozone_platform_wayland = true
       ozone_platform_x11 = true
     }