Clears the expired identities from the WebRTC identity cache.
We now check the creation time of an identity before returning for WebRTCIdentityStoreBackend::FindIdentity and remove the expired identity from the in-memory cache.
The expired identities are also removed from the on-disk database before it's loaded to the in-memory cache. 
The identities have 30-day expiration time by default for now. 

BUG=304398

Review URL: https://codereview.chromium.org/26082002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@227772 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 227772