gn: Support build directories outside the source tree.

tools/gn/function_exec_script.cc

Index: tools/gn/function_exec_script.cc
diff --git a/tools/gn/function_exec_script.cc b/tools/gn/function_exec_script.cc
index eb30a6ec399b64aac6ddad66441fdf03baf4aebf..8079a5573ed01a57f4bf7ec20756965cb2254f83 100644
--- a/tools/gn/function_exec_script.cc
+++ b/tools/gn/function_exec_script.cc
@@ -306,7 +306,8 @@ Value RunExecScript(Scope* scope,
   if (!args[0].VerifyTypeIs(Value::STRING, err))
     return Value();
   SourceFile script_source =
-      cur_dir.ResolveRelativeFile(args[0].string_value());
+      cur_dir.ResolveRelativeFile(args[0].string_value(),
+          scope->settings()->build_settings()->root_path());
   base::FilePath script_path = build_settings->GetFullPath(script_source);
   if (!build_settings->secondary_source_path().empty() &&
       !base::PathExists(script_path)) {
@@ -330,7 +331,8 @@ Value RunExecScript(Scope* scope,
         return Value();
       g_scheduler->AddGenDependency(
           build_settings->GetFullPath(cur_dir.ResolveRelativeFile(
-              dep.string_value())));
+              dep.string_value(),
+              scope->settings()->build_settings()->root_path())));
     }
   }