Reland "Expose can_trickle_ice_candidates on PeerConnection"

This reverts commit cb8c40138ca170f841bc45fa6771cdfc4b966e5f.

Reason for revert: Added missing default.

Original change's description:
> Revert "Expose can_trickle_ice_candidates on PeerConnection"
>
> This reverts commit c6a65c8866487c6adc0a7bb472d3bad9389501f9.
>
> Reason for revert: Breaks downstream due to missing default
>
> Original change's description:
> > Expose can_trickle_ice_candidates on PeerConnection
> >
> > Bug: chromium:708484
> > Change-Id: I9a40e75066341f0d9f965bd3718bfcb3f0459533
> > Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/169450
> > Commit-Queue: Harald Alvestrand <hta@webrtc.org>
> > Reviewed-by: Taylor <deadbeef@webrtc.org>
> > Cr-Commit-Position: refs/heads/master@{#30653}
>
> TBR=deadbeef@webrtc.org,hta@webrtc.org
>
> Change-Id: Iaa5b977c4237715a8a5127cf167cf6512a3f7059
> No-Presubmit: true
> No-Tree-Checks: true
> No-Try: true
> Bug: chromium:708484
> Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/169540
> Reviewed-by: Harald Alvestrand <hta@webrtc.org>
> Commit-Queue: Harald Alvestrand <hta@webrtc.org>
> Cr-Commit-Position: refs/heads/master@{#30655}

TBR=deadbeef@webrtc.org,hta@webrtc.org

Change-Id: I608da7781f158b4b02dd226d4dcd5615c4935fa8
Bug: chromium:708484
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/169541
Commit-Queue: Harald Alvestrand <hta@webrtc.org>
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#30656}
diff --git a/api/peer_connection_interface.h b/api/peer_connection_interface.h
index cc37dab..0ae47b2 100644
--- a/api/peer_connection_interface.h
+++ b/api/peer_connection_interface.h
@@ -1109,6 +1109,13 @@
 
   virtual IceGatheringState ice_gathering_state() = 0;
 
+  // Returns the current state of canTrickleIceCandidates per
+  // https://w3c.github.io/webrtc-pc/#attributes-1
+  virtual absl::optional<bool> can_trickle_ice_candidates() {
+    // TODO(crbug.com/708484): Remove default implementation.
+    return absl::nullopt;
+  }
+
   // Start RtcEventLog using an existing output-sink. Takes ownership of
   // |output| and passes it on to Call, which will take the ownership. If the
   // operation fails the output will be closed and deallocated. The event log