Fix GN win build.

tools/gn/secondary/third_party/nss/BUILD.gn

 # 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.
 
 import("//build/config/linux/pkg_config.gni")
 
 if (is_linux) {
   # This is a dependency on NSS with no libssl. On Linux we use a built-in SSL
   # library but the system NSS libraries. Non-Linux platforms using NSS use the
   # hermetic one in //third_party/nss.
@@ skipping 341 matching lines @@
 
     if (include_nss_root_certs) {
       deps += [ ":nssckbi" ]
     }
 
     if (component_mode == "shared_library") {
       if (is_mac) {
         cflags = [ "-all_load" ]
       } else if (is_win) {
         # Pass the def file to the linker.
-        ldflags += [ rebase_path("nss/exports_win.def", root_build_dir) ]
+        ldflags = [ rebase_path("nss/exports_win.def", root_build_dir) ]
       }
     }
 
     forward_dependent_configs_from = deps
   }
 
   config("nssckbi_config") {
     include_dirs = [ "nss/lib/ckfw/builtins" ]
   }
 
@@ skipping 777 matching lines @@
 
     deps = [
       ":nspr",
       "//third_party/sqlite",
     ]
 
     forward_dependent_configs_from = [ ":nspr" ]
   }
 }  # Windows/Mac/iOS.