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}

rtc_base/experiments/quality_scaling_experiment.h

@@ -10,7 +10,7 @@
 #ifndef RTC_BASE_EXPERIMENTS_QUALITY_SCALING_EXPERIMENT_H_
 #define RTC_BASE_EXPERIMENTS_QUALITY_SCALING_EXPERIMENT_H_
 
-#include "api/optional.h"
+#include "absl/types/optional.h"
 #include "api/video_codecs/video_encoder.h"
 #include "common_types.h"  // NOLINT(build/include)
 
@@ -44,10 +44,10 @@ class QualityScalingExperiment {
   static bool Enabled();
 
   // Returns settings from field trial.
-  static rtc::Optional<Settings> ParseSettings();
+  static absl::optional<Settings> ParseSettings();
 
   // Returns QpThresholds for the |codec_type|.
-  static rtc::Optional<VideoEncoder::QpThresholds> GetQpThresholds(
+  static absl::optional<VideoEncoder::QpThresholds> GetQpThresholds(
       VideoCodecType codec_type);
 
   // Returns parsed values. If the parsing fails, default values are returned.