AU: Implement exponential back off for 500 and 503 HTTP response codes.

Also refactors the automatic update checks into a separate
scheduler class and adds unit tests for them.

update_check_scheduler.cc                  59 /   59: 100.0%
update_check_scheduler.h                    1 /    1: 100.0%

Note: because the unit tests for this CL use the
UpdateAttempter class, the CL brings in several untested
modules into the test report reducing the overall test
coverage to ~82%.

BUG=2394
TEST=unit tests, gmerged on device and inspected logs

Change-Id: I078b1727b5338f6fc34e51f5e04a375518d63cef

Review URL: http://codereview.chromium.org/3215006
diff --git a/omaha_request_action.h b/omaha_request_action.h
index 6f30a32..7533d14 100644
--- a/omaha_request_action.h
+++ b/omaha_request_action.h
@@ -127,6 +127,8 @@
   void PerformAction();
   void TerminateProcessing();
 
+  int GetHTTPResponseCode() { return http_fetcher_->http_response_code(); }
+
   // Debugging/logging
   static std::string StaticType() { return "OmahaRequestAction"; }
   std::string Type() const { return StaticType(); }