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

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

This CL generated by running script with parameter 'rtc_base'
Then manually fix where Optional was used without rtc prefix (patchset#3)

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: I825f80cc8089747876ba6316d9e7c30e05716974
Reviewed-on: https://webrtc-review.googlesource.com/84585
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Commit-Queue: Danil Chapovalov <danilchap@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#23700}

p2p/base/fakepackettransport.h

@@ -88,10 +88,10 @@ class FakePacketTransport : public PacketTransportInternal {
 
   const CopyOnWriteBuffer* last_sent_packet() { return &last_sent_packet_; }
 
-  Optional<NetworkRoute> network_route() const override {
+  absl::optional<NetworkRoute> network_route() const override {
     return network_route_;
   }
-  void SetNetworkRoute(Optional<NetworkRoute> network_route) {
+  void SetNetworkRoute(absl::optional<NetworkRoute> network_route) {
     network_route_ = network_route;
   }
 
@@ -132,7 +132,7 @@ class FakePacketTransport : public PacketTransportInternal {
   bool writable_ = false;
   bool receiving_ = false;
 
-  Optional<NetworkRoute> network_route_;
+  absl::optional<NetworkRoute> network_route_;
 };
 
 }  // namespace rtc