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.cc b/update_attempter.cc
index 694e774..a1568a9 100644
--- a/update_attempter.cc
+++ b/update_attempter.cc
@@ -297,6 +297,12 @@
     // If the request is not an event, then it's the update-check.
     if (!omaha_request_action->IsEvent()) {
       http_response_code_ = omaha_request_action->GetHTTPResponseCode();
+      // Forward the server-dictated poll interval to the update check
+      // scheduler, if any.
+      if (update_check_scheduler_) {
+        update_check_scheduler_->set_poll_interval(
+            omaha_request_action->GetOutputObject().poll_interval);
+      }
     }
   }
   if (code != kActionCodeSuccess) {