Add not_needed function

tools/gn/args.cc

Index: tools/gn/args.cc
diff --git a/tools/gn/args.cc b/tools/gn/args.cc
index 0dcceecd5a5fe396402b1558b071a5f059650465..862e2e900a3aee53cfd2018b756bb51211880b65 100644
--- a/tools/gn/args.cc
+++ b/tools/gn/args.cc
@@ -6,6 +6,7 @@
 
 #include "base/sys_info.h"
 #include "build/build_config.h"
+#include "tools/gn/scheduler.h"
 #include "tools/gn/string_utils.h"
 #include "tools/gn/variables.h"
 
@@ -210,7 +211,7 @@ bool Args::DeclareArgs(const Scope::KeyValueMap& args,
       scope_to_set->SetValue(toolchain_override->first,
                              toolchain_override->second,
                              toolchain_override->second.origin());
-      scope_to_set->MarkUsed(arg.first);
+      scope_to_set->MarkUsed(arg.first, err);
       continue;
     }
 
@@ -220,12 +221,12 @@ bool Args::DeclareArgs(const Scope::KeyValueMap& args,
     if (override != overrides_.end()) {
       scope_to_set->SetValue(override->first, override->second,
                              override->second.origin());
-      scope_to_set->MarkUsed(override->first);
+      scope_to_set->MarkUsed(override->first, err);
       continue;
     }
 
     scope_to_set->SetValue(arg.first, arg.second, arg.second.origin());
-    scope_to_set->MarkUsed(arg.first);
+    scope_to_set->MarkUsed(arg.first, err);
   }
 
   return true;
@@ -369,12 +370,15 @@ void Args::SetSystemVarsLocked(Scope* dest) const {
 
   // Mark these variables used so the build config file can override them
   // without geting a warning about overwriting an unused variable.
-  dest->MarkUsed(variables::kHostCpu);
-  dest->MarkUsed(variables::kCurrentCpu);
-  dest->MarkUsed(variables::kTargetCpu);
-  dest->MarkUsed(variables::kHostOs);
-  dest->MarkUsed(variables::kCurrentOs);
-  dest->MarkUsed(variables::kTargetOs);
+  Err err;
+  dest->MarkUsed(variables::kHostCpu, &err);
+  dest->MarkUsed(variables::kCurrentCpu, &err);
+  dest->MarkUsed(variables::kTargetCpu, &err);
+  dest->MarkUsed(variables::kHostOs, &err);
+  dest->MarkUsed(variables::kCurrentOs, &err);
+  dest->MarkUsed(variables::kTargetOs, &err);
+  if (err.has_error())
+    g_scheduler->FailWithError(err);
 }
 
 void Args::ApplyOverridesLocked(const Scope::KeyValueMap& values,