Replace rtc::Optional with absl::optional in test and rtc_tools

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

This CL generated by running script with parameters 'test rtc_tools'

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: Ibb43c737f4c45fe300736382b0dd2d8ab32c6377
Reviewed-on: https://webrtc-review.googlesource.com/83944
Reviewed-by: Patrik Höglund <phoglund@webrtc.org>
Commit-Queue: Danil Chapovalov <danilchap@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#23642}

test/testsupport/fileutils.h

@@ -16,7 +16,7 @@
 #include <string>
 #include <vector>
 
-#include "api/optional.h"
+#include "absl/types/optional.h"
 
 namespace webrtc {
 namespace test {
@@ -81,7 +81,7 @@ std::string WorkingDir();
 // of strings with one element for each found file or directory. Each element is
 // a path created by prepending |dir| to the file/directory name. "." and ".."
 // are never added in the returned vector.
-rtc::Optional<std::vector<std::string>> ReadDirectory(std::string path);
+absl::optional<std::vector<std::string>> ReadDirectory(std::string path);
 
 // Creates a directory if it not already exists.
 // Returns true if successful. Will print an error message to stderr and return