Replace rtc::Optional with absl::optional in pc

This is a no-op change because rtc::Optional is an alias to absl::optional

This CL generated by running script with parameter 'pc'

find $@ -type f \( -name \*.h -o -name \*.cc \) \
-exec sed -i 's|rtc::Optional|absl::optional|g' {} \+ \
-exec sed -i 's|rtc::nullopt|absl::nullopt|g' {} \+ \
-exec sed -i 's|#include "api/optional.h"|#include "absl/types/optional.h"|' {} \+

find $@ -type f -name BUILD.gn \
-exec sed -r -i 's|"[\./api]*:optional"|"//third_party/abseil-cpp/absl/types:optional"|' {} \+;

git cl format

Bug: webrtc:9078
Change-Id: Ide3b9eb32df7f25991f898ac58fcb119c9f8ae12
Reviewed-on: https://webrtc-review.googlesource.com/84181
Reviewed-by: Per Kjellander <perkj@webrtc.org>
Commit-Queue: Danil Chapovalov <danilchap@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#23669}

pc/peerconnection_ice_unittest.cc

@@ -981,7 +981,7 @@ TEST_F(PeerConnectionIceConfigTest, SetStunCandidateKeepaliveInterval) {
   config.ice_candidate_pool_size = 1;
   CreatePeerConnection(config);
   ASSERT_NE(port_allocator_, nullptr);
-  rtc::Optional<int> actual_stun_keepalive_interval =
+  absl::optional<int> actual_stun_keepalive_interval =
       port_allocator_->stun_candidate_keepalive_interval();
   EXPECT_EQ(actual_stun_keepalive_interval.value_or(-1), 123);
   config.stun_candidate_keepalive_interval = 321;