AU: Implement server-dictated poll interval.

The server will need to include a PollInterval XML attribute in its
update check response. The requested interval is in seconds.

BUG=5984
TEST=unit tests, gmerged on device and tested with a modified dev server

Change-Id: I89d13f9f85d93bc141b74ae677cca813e3364fb5

Review URL: http://codereview.chromium.org/3275006
diff --git a/update_attempter.h b/update_attempter.h
index 6a81ef2..5df6af9 100644
--- a/update_attempter.h
+++ b/update_attempter.h
@@ -11,10 +11,10 @@
 #include <string>
 #include <vector>
 
+#include <base/time.h>
 #include <glib.h>
 #include <gtest/gtest_prod.h>  // for FRIEND_TEST
 
-#include "base/time.h"
 #include "update_engine/action_processor.h"
 #include "update_engine/download_action.h"
 #include "update_engine/omaha_request_params.h"
@@ -169,6 +169,7 @@
   // Pending error event, if any.
   scoped_ptr<OmahaEvent> error_event_;
 
+  // HTTP server response code from the last HTTP request action.
   int http_response_code_;
 
   // Current process priority.