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

net/base/keygen_handler_win.cc

Index: net/base/keygen_handler_win.cc
diff --git a/net/base/keygen_handler_win.cc b/net/base/keygen_handler_win.cc
index 44365fa136db239c01ec334d51e2d760c57a77a0..e7f10d6618feecd929ed24a1b1e50b440e73a0c5 100644
--- a/net/base/keygen_handler_win.cc
+++ b/net/base/keygen_handler_win.cc
@@ -183,7 +183,7 @@ std::string KeygenHandler::GenKeyAndSignChallenge() {
             CRYPT_SILENT | CRYPT_NEWKEYSET))
       break;
 
-    if (GetLastError() != NTE_BAD_KEYSET) {
+    if (GetLastError() != static_cast<DWORD>(NTE_BAD_KEYSET)) {
       LOG(ERROR) << "Keygen failed: Couldn't acquire a CryptoAPI provider "
                     "context: " << GetLastError();
       return std::string();