Uses ProxyConfigServiceLinux in the chromoting host to configure proxy.

net/proxy/proxy_config_service_mac.h

Index: net/proxy/proxy_config_service_mac.h
diff --git a/net/proxy/proxy_config_service_mac.h b/net/proxy/proxy_config_service_mac.h
index fe5ce30c0580afd7882ba0e7acd7be39773d8caa..6c11c6f0981d9abf7b0562fbdfe9aa4622d13599 100644
--- a/net/proxy/proxy_config_service_mac.h
+++ b/net/proxy/proxy_config_service_mac.h
@@ -20,10 +20,7 @@ class SingleThreadTaskRunner;
 
 namespace net {
 
-// TODO(sergeyu): This class needs to be exported because remoting code
-// creates it directly. Fix that and remove NET_EXPORT here.
-// crbug.com/125104
-class NET_EXPORT ProxyConfigServiceMac : public ProxyConfigService {
+class ProxyConfigServiceMac : public ProxyConfigService {
  public:
   // Constructs a ProxyConfigService that watches the Mac OS system settings.
   // This instance is expected to be operated and deleted on the same thread