Move MarkBootSuccessful to BootControlInterface.

Updating the boot flags to mark the current boot as successful is
platform-specific and part of the BootControlInterface's job. This
patch moves this to a new async method in this interface.

Bug: 24077637
Test: FEATURES=test emerge-link update_engine; cros flash in Chrome OS; tested on a dragonboard.

Change-Id: I23c3ed915dd8d2588a90d84b212bb04977957975

boot_control_chromeos.h

diff --git a/boot_control_chromeos.h b/boot_control_chromeos.h
index a7c269e..6f5497b 100644
--- a/boot_control_chromeos.h
+++ b/boot_control_chromeos.h
@@ -19,6 +19,7 @@
 
 #include <string>
 
+#include <base/callback.h>
 #include <gtest/gtest_prod.h>  // for FRIEND_TEST
 
 #include "update_engine/boot_control_interface.h"
@@ -47,6 +48,7 @@
                           std::string* device) const override;
   bool IsSlotBootable(BootControlInterface::Slot slot) const override;
   bool MarkSlotUnbootable(BootControlInterface::Slot slot) override;
+  bool MarkBootSuccessfulAsync(base::Callback<void(bool)> callback) override;
 
  private:
   friend class BootControlChromeOSTest;