Add Ozone configuration to facilitate management of external platforms

ui/ozone/BUILD.gn

Index: ui/ozone/BUILD.gn
diff --git a/ui/ozone/BUILD.gn b/ui/ozone/BUILD.gn
index 028ac6b19d62549477695a9c5d8725a60d56ec66..b5746becf558ba6342010efb6ecd64e0d2c904dd 100644
--- a/ui/ozone/BUILD.gn
+++ b/ui/ozone/BUILD.gn
@@ -4,6 +4,7 @@
 
 import("//build/config/ui.gni")
 import("//ui/ozone/ozone.gni")
+import("//ui/ozone/ozone_extra.gni")
 import("//testing/test.gni")
 
 assert(use_ozone)
@@ -11,13 +12,13 @@ assert(use_ozone)
 visibility = [ ":*" ]
 
 # The list of platforms that will be built.
-ozone_platforms = []
+ozone_platforms = ozone_external_platforms
 
 # Extra dependencies to pull into ui/ozone for built platforms.
-ozone_platform_deps = []
+ozone_platform_deps = ozone_external_platform_deps
 
 # Extra dependencies to pull into ozone_unittests for built platforms.
-ozone_platform_test_deps = []
+ozone_platform_test_deps = ozone_external_platform_test_deps
 
 if (ozone_platform_headless) {
   ozone_platforms += [ "headless" ]