Introduce CreateDataChannelOrError

Deprecate CreateDataChannel, and make it a simple wrapper function.

Bug: webrtc:12796
Change-Id: I053d75a264596ba87ca734a29df9241de93a80c3
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/219784
Reviewed-by: Xavier Lepaul‎ <xalep@webrtc.org>
Reviewed-by: Henrik Boström <hbos@webrtc.org>
Commit-Queue: Harald Alvestrand <hta@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#34130}

api/test/mock_peerconnectioninterface.h

@@ -100,8 +100,8 @@ class MockPeerConnectionInterface
               GetSctpTransport,
               (),
               (const override));
-  MOCK_METHOD(rtc::scoped_refptr<DataChannelInterface>,
-              CreateDataChannel,
+  MOCK_METHOD(RTCErrorOr<rtc::scoped_refptr<DataChannelInterface>>,
+              CreateDataChannelOrError,
               (const std::string&, const DataChannelInit*),
               (override));
   MOCK_METHOD(const SessionDescriptionInterface*,