Fixes a crash in WebRTCIdentityServiceHost
It previously assumes that WebRTCIdentityStore always out lives WebRTCIdentityServiceHost, which is not true. It's now holding a refptr of WebRTCIdentityStore instead of a raw pointer.
In addition, the identity request callback may be called after WebRTCIdentityServiceHost is gone. So now a WeakPtr of WebRTCIdentityServiceHost is bound to the completion callback.

BUG=322195

Review URL: https://codereview.chromium.org/84943009

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@237151 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 237151