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

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

This CL generated by running script with parameter 'api'
Then undo changes to optional target itself and optional_unittests

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: I44093da213369d6a502e33792c694f620f53b779
Reviewed-on: https://webrtc-review.googlesource.com/84621
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Commit-Queue: Danil Chapovalov <danilchap@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#23707}

api/datachannelinterface.h

@@ -23,7 +23,7 @@
 namespace webrtc {
 
 // C++ version of: https://www.w3.org/TR/webrtc/#idl-def-rtcdatachannelinit
-// TODO(deadbeef): Use rtc::Optional for the "-1 if unset" things.
+// TODO(deadbeef): Use absl::optional for the "-1 if unset" things.
 struct DataChannelInit {
   // Deprecated. Reliability is assumed, and channel will be unreliable if
   // maxRetransmitTime or MaxRetransmits is set.