Use bootstat to record x-started timestamp
Use bootstat to record x-started timestamp

BUG=none
TEST=platform_BootPerfServer and login_LoginSuccess

Review URL: http://codereview.chromium.org/3123002

chadversary/chromiumos.platform.login_manager,chadversary/chromiumos.platform.login_manager,joshfire/chromiumos-platform-login_manager,chadversary/chromiumos.platform.login_manager,joshfire/chromiumos-platform-login_manager,joshfire/chromiumos-platform-login_manager