Switch OmahaEvent's error_code to ActionExitCode.

Also, emit the errorcode attribute only for non-success events.
Added explicit unit tests for OmahaEvent.

BUG=560
TEST=unit tests, gmerged on device, forced update, looked at logs.

Review URL: http://codereview.chromium.org/3035007
diff --git a/update_attempter.cc b/update_attempter.cc
index b5d9644..f4b284f 100644
--- a/update_attempter.cc
+++ b/update_attempter.cc
@@ -116,18 +116,14 @@
   shared_ptr<OmahaRequestAction> download_started_action(
       new OmahaRequestAction(omaha_request_params_,
                              new OmahaEvent(
-                                 OmahaEvent::kTypeUpdateDownloadStarted,
-                                 OmahaEvent::kResultSuccess,
-                                 0),
+                                 OmahaEvent::kTypeUpdateDownloadStarted),
                              new LibcurlHttpFetcher));
   shared_ptr<DownloadAction> download_action(
       new DownloadAction(new LibcurlHttpFetcher));
   shared_ptr<OmahaRequestAction> download_finished_action(
       new OmahaRequestAction(omaha_request_params_,
                              new OmahaEvent(
-                                 OmahaEvent::kTypeUpdateDownloadFinished,
-                                 OmahaEvent::kResultSuccess,
-                                 0),
+                                 OmahaEvent::kTypeUpdateDownloadFinished),
                              new LibcurlHttpFetcher));
   shared_ptr<PostinstallRunnerAction> postinstall_runner_action_precommit(
       new PostinstallRunnerAction(true));
@@ -137,9 +133,7 @@
       new PostinstallRunnerAction(false));
   shared_ptr<OmahaRequestAction> update_complete_action(
       new OmahaRequestAction(omaha_request_params_,
-                             new OmahaEvent(OmahaEvent::kTypeUpdateComplete,
-                                            OmahaEvent::kResultSuccess,
-                                            0),
+                             new OmahaEvent(OmahaEvent::kTypeUpdateComplete),
                              new LibcurlHttpFetcher));
 
   download_action->set_delegate(this);