Send minimal HTTP headers for DNS-over-HTTPS requests.

Use a string constant for the User-Agent header and a wildcard for the
Accept-Language header. This reduces the ability of providers to group
traffic from a given user across connections but allows providers to
trace the queries back to Chrome in debugging efforts.

Bug: 878582
Change-Id: I9c51b7400640f7df7c0d51cb6290d6a37f564f4f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1689398
Commit-Queue: Katharine Daly <dalyk@google.com>
Reviewed-by: Eric Orth <ericorth@chromium.org>
Cr-Commit-Position: refs/heads/master@{#678521}

net/dns/dns_transaction.cc

@@ -46,6 +46,7 @@
 #include "net/dns/dns_session.h"
 #include "net/dns/dns_util.h"
 #include "net/dns/public/dns_protocol.h"
+#include "net/http/http_request_headers.h"
 #include "net/log/net_log.h"
 #include "net/log/net_log_capture_mode.h"
 #include "net/log/net_log_event_type.h"
@@ -348,6 +349,9 @@ class DnsHTTPAttempt : public DnsAttempt, public URLRequest::Delegate {
 
     HttpRequestHeaders extra_request_headers;
     extra_request_headers.SetHeader("Accept", kDnsOverHttpResponseContentType);
+    // Send minimal request headers where possible.
+    extra_request_headers.SetHeader(HttpRequestHeaders::kAcceptLanguage, "*");
+    extra_request_headers.SetHeader(HttpRequestHeaders::kUserAgent, "Chrome");
 
     DCHECK(url_request_context);
     request_ = url_request_context->CreateRequest(

net/dns/dns_transaction_unittest.cc

@@ -823,6 +823,16 @@ class DnsTransactionTest : public DnsTransactionTestBase,
     EXPECT_TRUE(request->extra_request_headers().GetHeader("Accept", &accept));
     EXPECT_EQ(accept, "application/dns-message");
 
+    std::string language;
+    EXPECT_TRUE(request->extra_request_headers().GetHeader("Accept-Language",
+                                                           &language));
+    EXPECT_EQ(language, "*");
+
+    std::string user_agent;
+    EXPECT_TRUE(
+        request->extra_request_headers().GetHeader("User-Agent", &user_agent));
+    EXPECT_EQ(user_agent, "Chrome");
+
     SocketDataProvider* provider = socket_factory_->mock_data().GetNext();
 
     if (doh_job_maker_)