GN Mac build work

tools/gn/args.cc

 // Copyright (c) 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "tools/gn/args.h"
 
 #include "build/build_config.h"
 #include "tools/gn/variables.h"
 
 #if defined(OS_WIN)
@@ skipping 117 matching lines @@
             "Each build arg should have one default value so we report it "
             "nicely in the\n\"gn args\" command. Please make this value "
             "constant.");
         return false;
       }
     } else {
       declared_arguments_.insert(*i);
     }
 
     // Only set on the current scope to the new value if it hasn't been already
-    // set.
-    if (!scope_to_set->GetValue(i->first))
+    // set. Mark the variable used so the build script can override it in
+    // certain cases without getting unused value errors.
+    if (!scope_to_set->GetValue(i->first)) {
       scope_to_set->SetValue(i->first, i->second, i->second.origin());
+      scope_to_set->MarkUsed(i->first);
+    }
   }
 
   return true;
 }
 
 bool Args::VerifyAllOverridesUsed(Err* err) const {
   base::AutoLock lock(lock_);
 
   for (Scope::KeyValueMap::const_iterator i = all_overrides_.begin();
        i != all_overrides_.end(); ++i) {
@@ skipping 86 matching lines @@
        i != values.end(); ++i)
     scope->SetValue(i->first, i->second, i->second.origin());
 }
 
 void Args::SaveOverrideRecord(const Scope::KeyValueMap& values) const {
   base::AutoLock lock(lock_);
   for (Scope::KeyValueMap::const_iterator i = values.begin();
        i != values.end(); ++i)
     all_overrides_[i->first] = i->second;
 }