Update from https://crrev.com/308331

mojo/public/python/BUILD.gn

Index: mojo/public/python/BUILD.gn
diff --git a/mojo/public/python/BUILD.gn b/mojo/public/python/BUILD.gn
index 51bd6eb8599aaf0bf67ed84503be83fb63f3bca9..70c2dca4f14bac66567033a0411457edb1933858 100644
--- a/mojo/public/python/BUILD.gn
+++ b/mojo/public/python/BUILD.gn
@@ -16,44 +16,40 @@ group("python") {
 # GYP version: mojo.gyp:mojo_python_system
 python_binary_module("system") {
   python_base_module = "mojo"
-  sources = [
+  cython_sources = [
     "mojo/c_core.pxd",
     "mojo/system.pyx",
   ]
-  configs = [ "../build/config:mojo_sdk" ]
   deps = [
     ":base",
     "../c/environment",
-    "../c/system",
+    "../c/system:for_shared_library",
     "../cpp/environment:standalone",
     "../cpp/system",
     "../cpp/utility",
     "../cpp/bindings:callback",
-    "../platform/native:system",
   ]
 }
 
 python_binary_module("system_impl") {
   python_base_module = "mojo"
-  sources = [
+  cython_sources = [
     "mojo/c_core.pxd",
     "mojo/c_environment.pxd",
     "mojo/system_impl.pyx",
   ]
-  additional_sources = [
+  sources = [
     "src/python_system_helper.cc",
     "src/python_system_helper.h",
   ]
-  configs = [ "../build/config:mojo_sdk" ]
   deps = [
     ":base",
     "../c/environment",
-    "../c/system",
+    "../c/system:for_shared_library",
     "../cpp/environment:standalone",
     "../cpp/system",
     "../cpp/utility",
     "../cpp/bindings:callback",
-    "../platform/native:system",
   ]
 }