cablev2: update tunnel server domain

The default tunnel server domain is switched to ua5v.com and, as a
protocol change, "cable." is prepended. The latter makes using a CNAME
easier because CNAMEs can't exist at a zone cut. (I.e. ua5v.com itself
cannot be a CNAME because it must have an SOA record, but cable.ua5v.com
could be a CNAME.)

BUG=1002262

Change-Id: Ia2a69ffab82f0ef065b813ffee58f16ca02bcf28
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2463808
Commit-Queue: Adam Langley <agl@chromium.org>
Reviewed-by: Martin Kreichgauer <martinkr@google.com>
Cr-Commit-Position: refs/heads/master@{#816686}

device/fido/cable/v2_authenticator.cc

@@ -72,7 +72,7 @@ constexpr net::NetworkTrafficAnnotationTag kTrafficAnnotation =
 // communication. This specifies a Google service and the short domain need is
 // necessary to fit within a BLE advert.
 constexpr uint32_t kTunnelServer = device::cablev2::tunnelserver::EncodeDomain(
-    "xyi3",
+    "ua5v",
     device::cablev2::tunnelserver::TLD::COM);
 
 struct MakeCredRequest {

device/fido/cable/v2_handshake.cc

@@ -93,7 +93,7 @@ namespace tunnelserver {
 std::string DecodeDomain(uint32_t domain) {
   static const char kBase32Chars[33] = "abcdefghijklmnopqrstuvwxyz234567";
 
-  std::string ret;
+  std::string ret = "cable.";
   ret.push_back(kBase32Chars[(domain >> 17) & 0x1f]);
   ret.push_back(kBase32Chars[(domain >> 12) & 0x1f]);
   ret.push_back(kBase32Chars[(domain >> 7) & 0x1f]);

device/fido/cable/v2_handshake_unittest.cc

@@ -22,7 +22,7 @@ TEST(CableV2Encoding, TunnelServerURLs) {
       tunnelserver::EncodeDomain("abcd", tunnelserver::TLD::NET);
   uint8_t tunnel_id[16] = {0};
   const GURL url = tunnelserver::GetNewTunnelURL(encoded, tunnel_id);
-  EXPECT_TRUE(url.spec().find("//abcd.net/") != std::string::npos) << url;
+  EXPECT_TRUE(url.spec().find("//cable.abcd.net/") != std::string::npos) << url;
 }
 
 TEST(CableV2Encoding, EIDToFromComponents) {