weaved: Hook up long-poll HTTP requests for /privet/v3/checkForUpdates

libweave now supports log-poll HTTP requests for device state change
notifications as described in:
https://developers.google.com/cloud-devices/v1/reference/local-api/check_for_updates

Made corresponding implementation changes on weaved side.

BUG: 23908251
Change-Id: I60f9ad67907645b4164a5e8cf716a0a31e760d72
diff --git a/buffet/webserv_client.cc b/buffet/webserv_client.cc
index 3fe5d0c..a66ed3f 100644
--- a/buffet/webserv_client.cc
+++ b/buffet/webserv_client.cc
@@ -135,6 +135,12 @@
   return https_port_;
 }
 
+base::TimeDelta WebServClient::GetRequestTimeout() const {
+  // TODO(avakulenko): obtain the timeout from web server.
+  // The current timeout is set to 1 minute.
+  return base::TimeDelta::FromSeconds(60);
+}
+
 brillo::Blob WebServClient::GetHttpsCertificateFingerprint() const {
   return certificate_;
 }