clang/win: Let some chromium_code targets build with -Wextra.

net/cert/test_root_certs_win.cc

Index: net/cert/test_root_certs_win.cc
diff --git a/net/cert/test_root_certs_win.cc b/net/cert/test_root_certs_win.cc
index a83ac0479054f2c3d4f0fcc000d1d1cc54dfb94e..36e7028d1a012bcd5c396050c2c1690e9d135b34 100644
--- a/net/cert/test_root_certs_win.cc
+++ b/net/cert/test_root_certs_win.cc
@@ -148,7 +148,7 @@ bool TestRootCerts::Add(X509Certificate* certificate) {
       CERT_STORE_ADD_NEW, NULL);
   if (!ok) {
     // If the certificate is already added, return successfully.
-    return GetLastError() == CRYPT_E_EXISTS;
+    return GetLastError() == static_cast<DWORD>(CRYPT_E_EXISTS);
   }
 
   empty_ = false;