[Telemetry] Properly chose an available device port.

tools/telemetry/telemetry/core/forwarders/__init__.py

Index: tools/telemetry/telemetry/core/forwarders/__init__.py
diff --git a/tools/telemetry/telemetry/core/forwarders/__init__.py b/tools/telemetry/telemetry/core/forwarders/__init__.py
index 71962736995fd4106da77aeb92f6bc3661f82e23..748f7bbb610033256e24c7f330dc7e9df369f909 100644
--- a/tools/telemetry/telemetry/core/forwarders/__init__.py
+++ b/tools/telemetry/telemetry/core/forwarders/__init__.py
@@ -33,7 +33,9 @@ class Forwarder(object):
 
   def __init__(self, port_pairs):
     assert port_pairs.http, 'HTTP port mapping is required.'
-    self._port_pairs = port_pairs
+    self._port_pairs = PortPairs(*[
+        PortPair(p.local_port, p.remote_port or p.local_port)
+        if p else None for p in port_pairs])
 
   @property
   def host_port(self):
@@ -44,6 +46,10 @@ class Forwarder(object):
     return '127.0.0.1'
 
   @property
+  def port_pairs(self):
+    return self._port_pairs
+
+  @property
   def url(self):
     assert self.host_ip and self.host_port
     return 'http://%s:%i' % (self.host_ip, self.host_port)