Fix accidentally positive HTTPS upgrade error code

ERR_DNS_NAME_HTTPS_ONLY was accidentally +809 instead of -809.

Also add some validation to ensure nobody accidentally makes a similar
invalid error in the future.

Bug: 1225776
Change-Id: I977733159de0f77e6701551d0948b73b22476114
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3098285
Reviewed-by: David Benjamin <davidben@chromium.org>
Commit-Queue: Eric Orth <ericorth@chromium.org>
Cr-Commit-Position: refs/heads/master@{#912367}

net/base/net_error_list.h

@@ -1000,4 +1000,4 @@ NET_ERROR(DNS_SECURE_RESOLVER_HOSTNAME_RESOLUTION_FAILED, -808)
 // DNS identified the request as disallowed for insecure connection (http/ws).
 // Error should be handled as if an HTTP redirect was received to redirect to
 // https or wss.
-NET_ERROR(DNS_NAME_HTTPS_ONLY, 809)
+NET_ERROR(DNS_NAME_HTTPS_ONLY, -809)

net/base/net_errors.cc

@@ -14,6 +14,12 @@
 
 namespace net {
 
+// Validate all error values in net_error_list.h are negative.
+#define NET_ERROR(label, value) \
+  static_assert(value < 0, "ERR_" #label " should be negative");
+#include "net/base/net_error_list.h"
+#undef NET_ERROR
+
 std::string ErrorToString(int error) {
   return "net::" + ErrorToShortString(error);
 }