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.h b/buffet/webserv_client.h
index 8b0ab7c..1c0d44c 100644
--- a/buffet/webserv_client.h
+++ b/buffet/webserv_client.h
@@ -57,6 +57,7 @@
 
   uint16_t GetHttpPort() const override;
   uint16_t GetHttpsPort() const override;
+  base::TimeDelta GetRequestTimeout() const override;
   std::vector<uint8_t> GetHttpsCertificateFingerprint() const override;
 
  private:
@@ -64,11 +65,6 @@
                  std::unique_ptr<libwebserv::Request> request,
                  std::unique_ptr<libwebserv::Response> response);
 
-  void OnResponse(std::unique_ptr<libwebserv::Response> response,
-                  int status_code,
-                  const std::string& data,
-                  const std::string& mime_type);
-
   void OnProtocolHandlerConnected(
       libwebserv::ProtocolHandler* protocol_handler);