Datachannel: Use absl::optional for maxRetransmits and maxRetransmitTime.

These parameters are nullable in the JS API.
This allows cleaner handling of "unset" vs "set" in Chrome.

Backwards compatibility note: Behavior should not change, even for users
who set the values explicitly to -1 in the DataChannelInit struct.
Those who try to read back the value will get a compile-time error.

Bug: chromium:854385
Change-Id: Ib488ca5f70bc24ba8b4a3f71b506434c4d2c60b2
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/131381
Reviewed-by: Henrik Boström <hbos@webrtc.org>
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Commit-Queue: Harald Alvestrand <hta@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#27507}

pc/peer_connection_integrationtest.cc

@@ -3207,6 +3207,7 @@ TEST_P(PeerConnectionIntegrationTest, RtpDataChannelsRejectedByCallee) {
       CreatePeerConnectionWrappersWithConfig(rtc_config_1, rtc_config_2));
   ConnectFakeSignaling();
   caller()->CreateDataChannel();
+  ASSERT_TRUE(caller()->data_channel() != nullptr);
   caller()->AddAudioVideoTracks();
   callee()->AddAudioVideoTracks();
   caller()->CreateAndSetAndSignalOffer();