crash-reporter: fix race condition between cron crash_sender and test invoked

Change-Id: I274072faffcb49b1910352cbcc5005e3144d9ab0

BUG=7765
TEST=CrashSender and UserCrash

Review URL: http://codereview.chromium.org/3748011
1 file changed
tree: 87ef86aa3ae73ff9778f9eafc6356ebe0ccb684a
  1. crash_reporter/