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/field_trial_parser_unittest.cc

@@ -43,14 +43,14 @@ enum class CustomEnum {
 // Providing a custom parser for an enum can make the trial string easier to
 // read, but also adds more code and makes the string more verbose.
 template <>
-rtc::Optional<CustomEnum> ParseTypedParameter<CustomEnum>(std::string str) {
+absl::optional<CustomEnum> ParseTypedParameter<CustomEnum>(std::string str) {
   if (str == "default")
     return CustomEnum::kDefault;
   else if (str == "red")
     return CustomEnum::kRed;
   else if (str == "blue")
     return CustomEnum::kBlue;
-  return rtc::nullopt;
+  return absl::nullopt;
 }
 
 TEST(FieldTrialParserTest, ParsesValidParameters) {
@@ -104,7 +104,7 @@ TEST(FieldTrialParserTest, IgnoresInvalid) {
   EXPECT_EQ(exp.hash.Get(), "a80");
 }
 TEST(FieldTrialParserTest, ParsesOptionalParameters) {
-  FieldTrialOptional<int> max_count("c", rtc::nullopt);
+  FieldTrialOptional<int> max_count("c", absl::nullopt);
   ParseFieldTrial({&max_count}, "");
   EXPECT_FALSE(max_count.Get().has_value());
   ParseFieldTrial({&max_count}, "c:10");