TurnCustomizer - an interface for modifying stun messages sent by TurnPort

This patch adds an interface that allows modification of stun messages
sent by TurnPort. A user can inject a TurnCustomizer on the RTCConfig
and the TurnCustomizer will be invoked by TurnPort before sending
message. This allows user to e.g add custom attributes as described
in rtf5389.

BUG=webrtc:8313

Change-Id: I6f4333e9f8ff7fd20f32677be19285f15e1180b6
Reviewed-on: https://webrtc-review.googlesource.com/7618
Reviewed-by: Guido Urdaneta <guidou@webrtc.org>
Reviewed-by: Taylor Brandstetter <deadbeef@webrtc.org>
Reviewed-by: Sami Kalliomäki <sakal@webrtc.org>
Commit-Queue: Jonas Oreland <jonaso@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#20233}

p2p/base/portallocator.h

@@ -25,6 +25,7 @@
 
 namespace webrtc {
 class MetricsObserverInterface;
+class TurnCustomizer;
 }
 
 namespace cricket {
@@ -362,7 +363,8 @@ class PortAllocator : public sigslot::has_slots<> {
   bool SetConfiguration(const ServerAddresses& stun_servers,
                         const std::vector<RelayServerConfig>& turn_servers,
                         int candidate_pool_size,
-                        bool prune_turn_ports);
+                        bool prune_turn_ports,
+                        webrtc::TurnCustomizer* turn_customizer = nullptr);
 
   const ServerAddresses& stun_servers() const { return stun_servers_; }
 
@@ -477,6 +479,10 @@ class PortAllocator : public sigslot::has_slots<> {
     metrics_observer_ = observer;
   }
 
+  webrtc::TurnCustomizer* turn_customizer() {
+    return turn_customizer_;
+  }
+
  protected:
   virtual PortAllocatorSession* CreateSessionInternal(
       const std::string& content_name,
@@ -512,6 +518,11 @@ class PortAllocator : public sigslot::has_slots<> {
   bool prune_turn_ports_ = false;
 
   webrtc::MetricsObserverInterface* metrics_observer_ = nullptr;
+
+  // Customizer for TURN messages.
+  // The instance is owned by application and will be shared among
+  // all TurnPort(s) created.
+  webrtc::TurnCustomizer* turn_customizer_ = nullptr;
 };
 
 }  // namespace cricket