dns_util: Make DohUpgradeEntry non-const when used with std::vector<>

This fixes the build with libstdc++ (with most other standard libraries
other than libc++, in fact) after commit f93a48e3 ("Allow upgrade to DoH
during automatic mode"):

../../../../../../usr/bin/../lib/gcc/x86_64-redhat-linux/8/../../../../include/c++/8/bits/stl_vector.h:351:7: error: static_assert failed due to requirement 'is_same<typename remove_cv<const DohUpgradeEntry>::type, const DohUpgradeEntry>::value' "std::vector must have a non-const, non-volatile value_type"
      static_assert(is_same<typename remove_cv<_Tp>::type, _Tp>::value,
      ^             ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../../base/no_destructor.h:77:28: note: in instantiation of template class 'std::vector<const net::(anonymous namespace)::DohUpgradeEntry, std::allocator<const net::(anonymous namespace)::DohUpgradeEntry> >' requested here
  alignas(T) char storage_[sizeof(T)];
                           ^
../../net/dns/dns_util.cc:147:7: note: in instantiation of template class 'base::NoDestructor<std::vector<const net::(anonymous namespace)::DohUpgradeEntry, std::allocator<const net::(anonymous namespace)::DohUpgradeEntry> > >' requested here
      upgradable_servers({
      ^
../../net/dns/dns_util.cc:230:36: error: invalid range expression of type 'const std::vector<const net::(anonymous namespace)::DohUpgradeEntry, std::allocator<const net::(anonymous namespace)::DohUpgradeEntry> >'; no viable 'begin' function available
    for (const auto& upgrade_entry : upgradable_servers) {
                                   ^ ~~~~~~~~~~~~~~~~~~

The C++ standard forbids containers of const elements. Callers of
GetDohUpgradeList() use it in a safe way anyway, and most of
DohUpgradeEntry's members are const.

Bug: 957519
Change-Id: I826a51823edb1184c0fae27105101e2894efe568
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1805636
Auto-Submit: Raphael Kubo da Costa <raphael.kubo.da.costa@intel.com>
Commit-Queue: Eric Orth <ericorth@chromium.org>
Reviewed-by: Eric Orth <ericorth@chromium.org>
Cr-Commit-Position: refs/heads/master@{#696834}

net/dns/dns_util.cc

@@ -139,11 +139,11 @@ struct DohUpgradeEntry {
   const DnsConfig::DnsOverHttpsServerConfig dns_over_https_config;
 };
 
-const std::vector<const DohUpgradeEntry>& GetDohUpgradeList() {
+const std::vector<DohUpgradeEntry>& GetDohUpgradeList() {
   // The provider names in these entries should be kept in sync with the
   // DohProviderId histogram suffix list in
   // tools/metrics/histograms/histograms.xml.
-  static const base::NoDestructor<std::vector<const DohUpgradeEntry>>
+  static const base::NoDestructor<std::vector<DohUpgradeEntry>>
       upgradable_servers({
           DohUpgradeEntry(
               "CleanBrowsingAdult",
@@ -222,8 +222,7 @@ const std::vector<const DohUpgradeEntry>& GetDohUpgradeList() {
 std::vector<const DohUpgradeEntry*> GetDohUpgradeEntriesFromNameservers(
     const std::vector<IPEndPoint>& dns_servers,
     const std::vector<std::string>& excluded_providers) {
-  const std::vector<const DohUpgradeEntry>& upgradable_servers =
-      GetDohUpgradeList();
+  const std::vector<DohUpgradeEntry>& upgradable_servers = GetDohUpgradeList();
   std::vector<const DohUpgradeEntry*> entries;
 
   for (const auto& server : dns_servers) {
@@ -417,8 +416,7 @@ std::vector<DnsConfig::DnsOverHttpsServerConfig>
 GetDohUpgradeServersFromDotHostname(
     const std::string& dot_server,
     const std::vector<std::string>& excluded_providers) {
-  const std::vector<const DohUpgradeEntry>& upgradable_servers =
-      GetDohUpgradeList();
+  const std::vector<DohUpgradeEntry>& upgradable_servers = GetDohUpgradeList();
   std::vector<DnsConfig::DnsOverHttpsServerConfig> doh_servers;
 
   if (dot_server.empty())
@@ -451,8 +449,7 @@ GetDohUpgradeServersFromNameservers(
 
 std::string GetDohProviderIdForHistogramFromDohConfig(
     const DnsConfig::DnsOverHttpsServerConfig& doh_server) {
-  const std::vector<const DohUpgradeEntry>& upgradable_servers =
-      GetDohUpgradeList();
+  const std::vector<DohUpgradeEntry>& upgradable_servers = GetDohUpgradeList();
   for (const auto& upgrade_entry : upgradable_servers) {
     if (doh_server.server_template ==
         upgrade_entry.dns_over_https_config.server_template) {