Add compile flag for enabling the compostior

sky/engine/config.gni

Index: sky/engine/config.gni
diff --git a/sky/engine/config.gni b/sky/engine/config.gni
index cd8b6ee10c6b3ce15e076b012aab441a9f9c085d..a924e3a122ec6fffbb58065a9bc0c56fcd04ddf6 100644
--- a/sky/engine/config.gni
+++ b/sky/engine/config.gni
@@ -27,6 +27,8 @@ declare_args() {
   sky_gc_plugin_dump_graph = false
 
   sky_asserts_always_on = false
+
+  sky_enable_compostior = false
 }
 
 # feature_defines_list ---------------------------------------------------------
@@ -50,6 +52,9 @@ if (sky_enable_oilpan) {
 if (sky_asserts_always_on) {
   feature_defines_list += [ "ENABLE_ASSERT=1" ]
 }
+if (sky_enable_compostior) {
+  feature_defines_list += [ "ENABLE_COMPOSITOR=1" ]
+}
 
 # feature_defines_string -------------------------------------------------------