Reduce absolute paths in generated ninja files

ppapi/native_client/BUILD.gn

Index: ppapi/native_client/BUILD.gn
diff --git a/ppapi/native_client/BUILD.gn b/ppapi/native_client/BUILD.gn
index eef25a26d78346ba698ecdea575fa3a3e40452e2..e06e4fadafd9b6076d22c255a55cd86b75de8c19 100644
--- a/ppapi/native_client/BUILD.gn
+++ b/ppapi/native_client/BUILD.gn
@@ -95,10 +95,10 @@ if (is_nacl) {
 
     script = "irt_debuglink.py"
     args = [
-      rebase_path(objcopy),
-      rebase_path(irt_debug),
-      rebase_path(irt_stripped),
-      rebase_path(irt_final),
+      rebase_path(objcopy, root_build_dir),
+      rebase_path(irt_debug, root_build_dir),
+      rebase_path(irt_stripped, root_build_dir),
+      rebase_path(irt_final, root_build_dir),
     ]
   }
 }