Revert of Implement RTCConfiguration.iceCandidatePoolSize. (patchset #7 id:120001 of https://codereview.webrtc.org/1956453003/ )

Reason for revert:
Breaks remoting_unittests. They defined their own operator== which conflicts with this one.

I'll remove the operator== in a roll CL. But until it's approved, I'm reverting this so the FYI bots will pass.

Original issue's description:
> Implement RTCConfiguration.iceCandidatePoolSize.
>
> It works by creating pooled PortAllocatorSessions which can be picked up
> by a P2PTransportChannel when needed (after a local description is set).
>
> This can optimize candidate gathering time when there is some time between
> creating a PeerConnection and setting a local description.
>
> R=pthatcher@webrtc.org
>
> Committed: https://chromium.googlesource.com/external/webrtc/+/48e9d05f510b1616c81303944008f75825971802

TBR=pthatcher@webrtc.org,honghaiz@webrtc.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true

Review-Url: https://codereview.webrtc.org/1972043004
Cr-Commit-Position: refs/heads/master@{#12709}
diff --git a/webrtc/api/peerconnectioninterface.h b/webrtc/api/peerconnectioninterface.h
index fe0dc1d..94d2c00 100644
--- a/webrtc/api/peerconnectioninterface.h
+++ b/webrtc/api/peerconnectioninterface.h
@@ -270,30 +270,42 @@
     static const int kAudioJitterBufferMaxPackets = 50;
     // TODO(pthatcher): Rename this ice_transport_type, but update
     // Chromium at the same time.
-    IceTransportsType type = kAll;
+    IceTransportsType type;
     // TODO(pthatcher): Rename this ice_servers, but update Chromium
     // at the same time.
     IceServers servers;
-    BundlePolicy bundle_policy = kBundlePolicyBalanced;
-    RtcpMuxPolicy rtcp_mux_policy = kRtcpMuxPolicyNegotiate;
-    TcpCandidatePolicy tcp_candidate_policy = kTcpCandidatePolicyEnabled;
-    int audio_jitter_buffer_max_packets = kAudioJitterBufferMaxPackets;
-    bool audio_jitter_buffer_fast_accelerate = false;
-    int ice_connection_receiving_timeout = kUndefined;         // ms
-    int ice_backup_candidate_pair_ping_interval = kUndefined;  // ms
-    ContinualGatheringPolicy continual_gathering_policy = GATHER_ONCE;
+    BundlePolicy bundle_policy;
+    RtcpMuxPolicy rtcp_mux_policy;
+    TcpCandidatePolicy tcp_candidate_policy;
+    int audio_jitter_buffer_max_packets;
+    bool audio_jitter_buffer_fast_accelerate;
+    int ice_connection_receiving_timeout;         // ms
+    int ice_backup_candidate_pair_ping_interval;  // ms
+    ContinualGatheringPolicy continual_gathering_policy;
     std::vector<rtc::scoped_refptr<rtc::RTCCertificate>> certificates;
-    bool prioritize_most_likely_ice_candidate_pairs = false;
+    bool prioritize_most_likely_ice_candidate_pairs;
     struct cricket::MediaConfig media_config;
     // Flags corresponding to values set by constraint flags.
     // rtc::Optional flags can be "missing", in which case the webrtc
     // default applies.
-    bool disable_ipv6 = false;
-    bool enable_rtp_data_channel = false;
+    bool disable_ipv6;
+    bool enable_rtp_data_channel;
     rtc::Optional<int> screencast_min_bitrate;
     rtc::Optional<bool> combined_audio_video_bwe;
     rtc::Optional<bool> enable_dtls_srtp;
-    int ice_candidate_pool_size = 0;
+    RTCConfiguration()
+        : type(kAll),
+          bundle_policy(kBundlePolicyBalanced),
+          rtcp_mux_policy(kRtcpMuxPolicyNegotiate),
+          tcp_candidate_policy(kTcpCandidatePolicyEnabled),
+          audio_jitter_buffer_max_packets(kAudioJitterBufferMaxPackets),
+          audio_jitter_buffer_fast_accelerate(false),
+          ice_connection_receiving_timeout(kUndefined),
+          ice_backup_candidate_pair_ping_interval(kUndefined),
+          continual_gathering_policy(GATHER_ONCE),
+          prioritize_most_likely_ice_candidate_pairs(false),
+          disable_ipv6(false),
+          enable_rtp_data_channel(false) {}
   };
 
   struct RTCOfferAnswerOptions {