Fix an issue in DtlsIdentityStore when the store is destroyed before the worker thread task returns.

BUG=crbug/464995
R=pthatcher@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/42659004

Cr-Commit-Position: refs/heads/master@{#8689}
git-svn-id: http://webrtc.googlecode.com/svn/trunk@8689 4adac7df-926f-26a2-2b94-8c16560cd09d
4 files changed