Change the gating of surfacing candidates on ICE transport type change

from a field trial to RTCConfiguration.

The test coverage is also expanded for the underlying feature.

Bug: None
Change-Id: Ic9c1362867e4a956c5453be7a9355083b6a442f5
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/138980
Reviewed-by: Steve Anton <steveanton@webrtc.org>
Reviewed-by: Alex Glaznev <glaznev@webrtc.org>
Reviewed-by: Kári Helgason <kthelgason@webrtc.org>
Reviewed-by: Amit Hilbuch <amithi@webrtc.org>
Commit-Queue: Qingsi Wang <qingsi@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#28143}

sdk/objc/api/peerconnection/RTCConfiguration.h

@@ -140,6 +140,18 @@ RTC_OBJC_EXPORT
  */
 @property(nonatomic, assign) BOOL shouldPresumeWritableWhenFullyRelayed;
 
+/* This flag is only effective when |continualGatheringPolicy| is
+ * RTCContinualGatheringPolicyGatherContinually.
+ *
+ * If YES, after the ICE transport type is changed such that new types of
+ * ICE candidates are allowed by the new transport type, e.g. from
+ * RTCIceTransportPolicyRelay to RTCIceTransportPolicyAll, candidates that
+ * have been gathered by the ICE transport but not matching the previous
+ * transport type and as a result not observed by PeerConnectionDelegateAdapter,
+ * will be surfaced to the delegate.
+ */
+@property(nonatomic, assign) BOOL shouldSurfaceIceCandidatesOnIceTransportTypeChanged;
+
 /** If set to non-nil, controls the minimal interval between consecutive ICE
  *  check packets.
  */