Add update reboot metric to the update engine.

This change add the Installer.UpdateNumReboots metric.

This records the number of reboots that occurred while an update was being
attempted. It uses a marker file stored in tmp to discover whether or not
it's already recorded the reboot.

BUG=chromium:226766
TEST=Unittests | ran an update on a test machine and rebooted/resumed and
checked about:histograms to confirm numbers. Also restart update-engine to
verify it didn't double count that.

Change-Id: I5d2af9d5b62a9d974c7c6243a89cb3359051b650
Reviewed-on: https://gerrit.chromium.org/gerrit/47710
Tested-by: Chris Sosa <sosa@chromium.org>
Commit-Queue: Chris Sosa <sosa@chromium.org>
Reviewed-by: Chris Sosa <sosa@chromium.org>
diff --git a/postinstall_runner_action.cc b/postinstall_runner_action.cc
index 934b461..802a2cd 100644
--- a/postinstall_runner_action.cc
+++ b/postinstall_runner_action.cc
@@ -8,7 +8,7 @@
 #include <stdlib.h>
 #include <vector>
 
-#include "update_engine/constants.h"
+#include "update_engine/action_processor.h"
 #include "update_engine/subprocess.h"
 #include "update_engine/utils.h"