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}

sdk/objc/api/peerconnection/RTCDataChannelConfiguration.mm

@@ -33,7 +33,7 @@
 }
 
 - (int)maxPacketLifeTime {
-  return _nativeDataChannelInit.maxRetransmitTime;
+  return *_nativeDataChannelInit.maxRetransmitTime;
 }
 
 - (void)setMaxPacketLifeTime:(int)maxPacketLifeTime {
@@ -41,7 +41,11 @@
 }
 
 - (int)maxRetransmits {
-  return _nativeDataChannelInit.maxRetransmits;
+  if (_nativeDataChannelInit.maxRetransmits) {
+    return *_nativeDataChannelInit.maxRetransmits;
+  } else {
+    return -1;
+  }
 }
 
 - (void)setMaxRetransmits:(int)maxRetransmits {