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}
diff --git a/pc/peerconnection_rtp_unittest.cc b/pc/peerconnection_rtp_unittest.cc
index d3ae2cb..ca2d3b1 100644
--- a/pc/peerconnection_rtp_unittest.cc
+++ b/pc/peerconnection_rtp_unittest.cc
@@ -791,10 +791,10 @@
   auto caller = CreatePeerConnection();
 
   auto transceiver = caller->AddTransceiver(cricket::MEDIA_TYPE_AUDIO);
-  EXPECT_EQ(rtc::nullopt, transceiver->mid());
+  EXPECT_EQ(absl::nullopt, transceiver->mid());
   EXPECT_FALSE(transceiver->stopped());
   EXPECT_EQ(RtpTransceiverDirection::kSendRecv, transceiver->direction());
-  EXPECT_EQ(rtc::nullopt, transceiver->current_direction());
+  EXPECT_EQ(absl::nullopt, transceiver->current_direction());
 }
 
 // Test that adding a transceiver with the audio kind creates an audio sender