Remove duplication between Mandoline and mojo_runner main functions.

mojo/runner/BUILD.gn

Index: mojo/runner/BUILD.gn
diff --git a/mojo/runner/BUILD.gn b/mojo/runner/BUILD.gn
index 803b4ff101d1715d0967ff2207124138eddd6539..c70eee01a7cb6c50646fff3b1512714892b14291 100644
--- a/mojo/runner/BUILD.gn
+++ b/mojo/runner/BUILD.gn
@@ -44,8 +44,6 @@ source_set("mojo_runner_lib") {
 
   if (!is_android) {
     sources += [
-      "desktop/launcher_process.cc",
-      "desktop/launcher_process.h",
       "desktop/main.cc",
       "register_local_aliases.cc",
       "register_local_aliases.h",
@@ -174,6 +172,13 @@ source_set("lib") {
       "//components/mus/gles2",
       "//mojo/application/public/cpp:content_handler",
     ]
+  } else {
+    sources += [
+      "desktop/launcher_process.cc",
+      "desktop/launcher_process.h",
+      "desktop/main_helper.cc",
+      "desktop/main_helper.h",
+    ]
   }
 
   # This target includes some files behind #ifdef OS... guards. Since gn is not