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.cc b/examples/provider/event_http_server.cc
index bb55c50..e0ecea6 100644
--- a/examples/provider/event_http_server.cc
+++ b/examples/provider/event_http_server.cc
@@ -183,6 +183,10 @@
   return 7781;
 }
 
+base::TimeDelta HttpServerImpl::GetRequestTimeout() const {
+  return base::TimeDelta::Max();
+}
+
 std::vector<uint8_t> HttpServerImpl::GetHttpsCertificateFingerprint() const {
   return cert_fingerprint_;
 }