Revert of Replacing DtlsIdentityStoreInterface with RTCCertificateGeneratorInterface. (patchset #2 id:20001 of )

Reason for revert:
There are more CreatePeerConnection calls than I anticipated/had found in Chromium, like remoting/protocol/ Reverting due to broken Chromium FYI bots.

Original issue's description:
> Replacing DtlsIdentityStoreInterface with RTCCertificateGeneratorInterface.
> The store was used in WebRtcSessionDescriptionFactory to generate certificates,
> now a generator is used instead (new API). PeerConnection[Factory][Interface],
> and WebRtcSession are updated to pass generators all the way down to the
> WebRtcSessionDescriptionFactory instead of stores.
> The webrtc implementation of a generator, RTCCertificateGenerator, is used as
> the default generator ( instead of the webrtc
> implementation of a store, DtlsIdentityStoreImpl.
>   The generator is fully parameterized and does not generate RSA-1024 unless you
> ask for it (which makes sense not to do beforehand since ECDSA is now default).
> The store was not fully parameterized (known filed bug).
> The "top" layer, PeerConnectionFactoryInterface::CreatePeerConnection, is
> updated to take a generator instead of a store. But as to not break Chromium,
> the old function signature taking a store is kept. It is implemented to invoke
> the generator version by wrapping the store in an
> RTCCertificateGeneratorStoreWrapper. As soon as Chromium is updated to use the
> new function signature we can remove the old CreatePeerConnection.
>   Due to having multiple CreatePeerConnection signatures, some calling places
> are updated to resolve the ambiguity introduced.
> BUG=webrtc:5707, webrtc:5708
> Committed:,,
# Skipping CQ checks because original CL landed less than 1 days ago.
BUG=webrtc:5707, webrtc:5708

Cr-Original-Commit-Position: refs/heads/master@{#12948}
Cr-Mirrored-Commit: d7973ccdb54307ead8f9defba5ad77f929e36153
1 file changed