[Ozone] Add out of tree platforms build support

Introduced ozone_extra_path build argument. It points to additional
platforms file to include while building ozone platforms. It points to
//ui/ozone/ozone_extra.gni by default for backward compatibility.

Bug: 918118
Change-Id: If6db25b3693f319955fdec54d9f91d47422a6e38
Reviewed-on: https://chromium-review.googlesource.com/c/1391687Reviewed-by: Michael Spang <spang@chromium.org>
Commit-Queue: Michael Spang <spang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#619679}

ui/ozone/BUILD.gn

@@ -7,7 +7,6 @@ import("//build/config/ui.gni")
 import("//gpu/vulkan/features.gni")
 import("//testing/test.gni")
 import("//ui/ozone/ozone.gni")
-import("//ui/ozone/ozone_extra.gni")
 
 assert(use_ozone)
 
@@ -133,6 +132,9 @@ jumbo_component("ozone_base") {
     "//ui/ozone/public/interfaces",
     "//ui/ozone/platform/*",
   ]
+
+  # Out of tree platforms can depend on this.
+  visibility += ozone_external_platform_visibility
 }
 
 source_set("platform") {
@@ -187,6 +189,9 @@ source_set("platform") {
 
   # This is used for platform tests.
   visibility += [ "//ui/ozone/platform/*" ]
+
+  # This can be used for out of tree platforms tests.
+  visibility += ozone_external_platform_visibility
 }
 
 jumbo_component("ozone") {

ui/ozone/common/BUILD.gn

@@ -39,4 +39,7 @@ source_set("common") {
   ]
 
   visibility = [ "//ui/ozone/platform/*" ]
+
+  # Out of tree platforms can depend on this.
+  visibility += ozone_external_platform_visibility
 }

ui/ozone/ozone.gni

@@ -6,6 +6,10 @@ import("//build/config/chromecast_build.gni")
 import("//build/config/ui.gni")
 
 declare_args() {
+  # Ozone extra platforms file path. Can be overridden to build out of
+  # tree ozone platforms.
+  ozone_extra_path = "//ui/ozone/ozone_extra.gni"
+
   # Select platforms automatically. Turn this off for manual control.
   ozone_auto_platforms = use_ozone
 }
@@ -79,6 +83,13 @@ declare_args() {
   }
 }
 
+import(ozone_extra_path)
+
+_ozone_extra_directory = get_path_info(ozone_extra_path, "dir")
+
+# Extra paths to add to targets visibility list.
+ozone_external_platform_visibility = [ "$_ozone_extra_directory/*" ]
+
 assert(use_ozone || !(ozone_platform_cast || ozone_platform_gbm ||
                           ozone_platform_headless || ozone_platform_x11 ||
                           ozone_platform_wayland || ozone_platform_windows ||