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/peerconnectioninterface_unittest.cc

@@ -2487,7 +2487,7 @@ TEST_P(PeerConnectionInterfaceTest, SetConfigurationChangesPruneTurnPortsFlag) {
 // require a very complex set of mocks.
 TEST_P(PeerConnectionInterfaceTest, SetConfigurationChangesIceCheckInterval) {
   PeerConnectionInterface::RTCConfiguration config;
-  config.ice_check_min_interval = rtc::nullopt;
+  config.ice_check_min_interval = absl::nullopt;
   CreatePeerConnection(config, nullptr);
   config = pc_->GetConfiguration();
   config.ice_check_min_interval = 100;