Revert of Replacing DtlsIdentityStoreInterface with RTCCertificateGeneratorInterface. (patchset #2 id:20001 of https://codereview.webrtc.org/2013523002/ )

Reason for revert:
There are more CreatePeerConnection calls than I anticipated/had found in Chromium, like remoting/protocol/webrtc_transport.cc. 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 (peerconnectionfactory.cc:189) 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
> R=phoglund@webrtc.org, tommi@webrtc.org
> TBR=tkchin@webrc.org
>
> Committed: 400781a209

TBR=tkchin@webrtc.org,tommi@webrtc.org,phoglund@webrtc.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=webrtc:5707, webrtc:5708

Review-Url: https://codereview.webrtc.org/2020633002
Cr-Commit-Position: refs/heads/master@{#12948}

webrtc/sdk/objc/Framework/Classes/RTCPeerConnection.mm

@@ -226,12 +226,11 @@ void PeerConnectionDelegateAdapter::OnIceCandidatesRemoved(
     std::unique_ptr<webrtc::MediaConstraints> nativeConstraints =
         constraints.nativeConstraints;
     _peerConnection =
-        factory.nativeFactory->CreatePeerConnection(
-            *config,
-            nativeConstraints.get(),
-            nullptr,
-            std::unique_ptr<rtc::RTCCertificateGeneratorInterface>(),
-            _observer.get());
+        factory.nativeFactory->CreatePeerConnection(*config,
+                                                    nativeConstraints.get(),
+                                                    nullptr,
+                                                    nullptr,
+                                                    _observer.get());
     _localStreams = [[NSMutableArray alloc] init];
     _delegate = delegate;
   }