Moved declare_args for enable_oilpan from Source/config.gni to public/features.gni

Source/config.gni

Index: Source/config.gni
diff --git a/Source/config.gni b/Source/config.gni
index 4beeda6fe09784caec7d67856660ba1db27c2910..9e308d9f08f1cf4e310877bce533a590e7d22bda 100644
--- a/Source/config.gni
+++ b/Source/config.gni
@@ -3,6 +3,7 @@
 # found in the LICENSE file.
 
 import("//build/config/ui.gni")
+import("//third_party/WebKit/public/features.gni")
 
 if (is_android) {
   import("//build/config/android/config.gni")
@@ -18,9 +19,6 @@ if (cpu_arch == "arm") {
 }
 
 declare_args() {
-  # Enables the Oilpan garbage-collection infrastructure.
-  enable_oilpan = false
-
   # Set to true to enable the clang plugin that checks the usage of the Blink
   # garbage-collection infrastructure during compilation.
   blink_gc_plugin = false