[Nearby] Add location hint for WebRTC.

Because WebRTC is geo-sharded, both peer devices must specify the same
locale / "location hint" in order to signal each other via Tachyon.

The location hint is in ISO 3166-1 alpha-2 format:
  https://en.wikipedia.org/wiki/ISO_3166-1_alpha-2

Bug: 1134210
Change-Id: Ic42c7139bd2ac96de8d6ffebe5c15bb70be0c9aa
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2447096Reviewed-by: James Vecore <vecore@google.com>
Reviewed-by: Ryan Hansberry <hansberry@chromium.org>
Commit-Queue: Michael Hansen <hansenmichael@google.com>
Cr-Commit-Position: refs/heads/master@{#814880}

chrome/browser/nearby_sharing/instantmessaging/proto/instantmessaging.proto

@@ -36,10 +36,35 @@ message IdType {
   }
 }
 
+message LocationStandard {
+  enum Format {
+    UNKNOWN = 0;
+    // E164 country codes:
+    // https://en.wikipedia.org/wiki/List_of_country_calling_codes
+    // e.g. +1 for USA
+    E164_CALLING = 1;
+
+    // ISO 3166-1 alpha-2 country codes:
+    // https://en.wikipedia.org/wiki/ISO_3166-1_alpha-2
+    ISO_3166_1_ALPHA_2 = 2;
+  }
+}
+
+// LocationHint is used to specify a location as well as format.
+message LocationHint {
+  // Location is the location, provided in the format specified by format.
+  string location = 1;
+
+  // the format of location.
+  LocationStandard.Format format = 2;
+}
+
 message Id {
   IdType.Type type = 1;
   string id = 2;
   string app = 3;
+  string country_code = 4 [deprecated = true];
+  LocationHint location_hint = 5;
 }
 
 message ApiVersion {

chrome/browser/nearby_sharing/webrtc_signaling_messenger.cc

@@ -5,6 +5,7 @@
 #include "chrome/browser/nearby_sharing/webrtc_signaling_messenger.h"
 
 #include "base/bind_helpers.h"
+#include "base/i18n/timezone.h"
 #include "base/token.h"
 #include "chrome/browser/nearby_sharing/instantmessaging/proto/instantmessaging.pb.h"
 
@@ -16,6 +17,13 @@ constexpr int kMajorVersion = 1;
 constexpr int kMinorVersion = 24;
 constexpr int kPointVersion = 0;
 
+void BuildLocationHint(
+    chrome_browser_nearby_sharing_instantmessaging::LocationHint* hint) {
+  hint->set_location(base::CountryCodeForCurrentTimezone());
+  hint->set_format(chrome_browser_nearby_sharing_instantmessaging::
+                       LocationStandard_Format_ISO_3166_1_ALPHA_2);
+}
+
 void BuildId(chrome_browser_nearby_sharing_instantmessaging::Id* req_id,
              const std::string& id) {
   DCHECK(req_id);
@@ -23,6 +31,7 @@ void BuildId(chrome_browser_nearby_sharing_instantmessaging::Id* req_id,
   req_id->set_app(kAppName);
   req_id->set_type(
       chrome_browser_nearby_sharing_instantmessaging::IdType::NEARBY_ID);
+  BuildLocationHint(req_id->mutable_location_hint());
 }
 
 void BuildHeader(