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/channel.cc b/pc/channel.cc
index 8ee2ba5..713cd39 100644
--- a/pc/channel.cc
+++ b/pc/channel.cc
@@ -363,7 +363,7 @@
 }
 
 void BaseChannel::OnNetworkRouteChanged(
-    rtc::Optional<rtc::NetworkRoute> network_route) {
+    absl::optional<rtc::NetworkRoute> network_route) {
   RTC_DCHECK(network_thread_->IsCurrent());
   rtc::NetworkRoute new_route;
   if (network_route) {