Add support for long-poll HTTP request at /privet/v3/checkForUpdates

Added state/command definition notification mechanism through a long
poll HTTP request as described in:
https://developers.google.com/cloud-devices/v1/reference/local-api/check_for_updates

BUG: 23908251
Change-Id: I5285403f35f23a310404d3324a7a255b77211264
Reviewed-on: https://weave-review.googlesource.com/1533
Reviewed-by: Vitaly Buka <vitalybuka@google.com>
diff --git a/examples/provider/event_http_server.h b/examples/provider/event_http_server.h
index e51c6f4..950e536 100644
--- a/examples/provider/event_http_server.h
+++ b/examples/provider/event_http_server.h
@@ -34,6 +34,7 @@
                               const RequestHandlerCallback& callback) override;
   uint16_t GetHttpPort() const override;
   uint16_t GetHttpsPort() const override;
+  base::TimeDelta GetRequestTimeout() const override;
   std::vector<uint8_t> GetHttpsCertificateFingerprint() const override;
 
  private: