Revert 227772 "Clears the expired identities from the WebRTC ide..."

This fails on ASAN tests:
http://build.chromium.org/p/chromium.memory/builders/Linux%20ASAN%20Tests%20%282%29/builds/18341/steps/content_unittests/logs/stdio
==23976==ERROR: AddressSanitizer: heap-use-after-free on address 0x60b00002ae68 at pc 0x2281510 bp 0x7fff24edac90 sp 0x7fff24edac88
READ of size 8 at 0x60b00002ae68 thread T0
    #0 0x228150f in content::WebRTCIdentityStoreBackend::FindIdentity(GURL const&, std::string const&, std::string const&, base::Callback<void (int, std::string const&, std::string const&)> const&) content/browser/media/webrtc_identity_store_backend.cc:225
    #1 0x228340f in content::WebRTCIdentityStoreBackend::OnLoaded(scoped_ptr<std::map<content::WebRTCIdentityStoreBackend::IdentityKey, content::WebRTCIdentityStoreBackend::Identity, std::less<content::WebRTCIdentityStoreBackend::IdentityKey>, std::allocator<std::pair<content::WebRTCIdentityStoreBackend::IdentityKey const, content::WebRTCIdentityStoreBackend::Identity> > >, base::DefaultDeleter<std::map<content::WebRTCIdentityStoreBackend::IdentityKey, content::WebRTCIdentityStoreBackend::Identity, std::less<content::WebRTCIdentityStoreBackend::IdentityKey>, std::allocator<std::pair<content::WebRTCIdentityStoreBackend::IdentityKey const, content::WebRTCIdentityStoreBackend::Identity> > > > >) content/browser/media/webrtc_identity_store_backend.cc:353
    #2 0x2289f53 in Run base/bind_internal.h:190
....

> 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

TBR=jiayl@chromium.org

Review URL: https://codereview.chromium.org/26567009

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@227776 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 227776