Replace rtc::Optional with absl::optional in audio, call and video

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

This CL generated by running script with parameters 'audio call video':
#!/bin/bash
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: I02c5db956846a88a268a300ba086703a02d62e36
Reviewed-on: https://webrtc-review.googlesource.com/83722
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Commit-Queue: Danil Chapovalov <danilchap@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#23628}
diff --git a/call/syncable.h b/call/syncable.h
index a97990b..a914793 100644
--- a/call/syncable.h
+++ b/call/syncable.h
@@ -16,7 +16,7 @@
 
 #include <stdint.h>
 
-#include "api/optional.h"
+#include "absl/types/optional.h"
 
 namespace webrtc {
 
@@ -34,7 +34,7 @@
   virtual ~Syncable();
 
   virtual int id() const = 0;
-  virtual rtc::Optional<Info> GetInfo() const = 0;
+  virtual absl::optional<Info> GetInfo() const = 0;
   virtual uint32_t GetPlayoutTimestamp() const = 0;
   virtual void SetMinimumPlayoutDelay(int delay_ms) = 0;
 };