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}
diff --git a/rtc_base/string_to_number.cc b/rtc_base/string_to_number.cc
index fe17f34..06ac9e9 100644
--- a/rtc_base/string_to_number.cc
+++ b/rtc_base/string_to_number.cc
@@ -18,7 +18,7 @@
 namespace rtc {
 namespace string_to_number_internal {
 
-rtc::Optional<signed_type> ParseSigned(const char* str, int base) {
+absl::optional<signed_type> ParseSigned(const char* str, int base) {
   RTC_DCHECK(str);
   if (isdigit(str[0]) || str[0] == '-') {
     char* end = nullptr;
@@ -28,10 +28,10 @@
       return value;
     }
   }
-  return rtc::nullopt;
+  return absl::nullopt;
 }
 
-rtc::Optional<unsigned_type> ParseUnsigned(const char* str, int base) {
+absl::optional<unsigned_type> ParseUnsigned(const char* str, int base) {
   RTC_DCHECK(str);
   if (isdigit(str[0]) || str[0] == '-') {
     // Explicitly discard negative values. std::strtoull parsing causes unsigned
@@ -45,7 +45,7 @@
       return value;
     }
   }
-  return rtc::nullopt;
+  return absl::nullopt;
 }
 
 }  // namespace string_to_number_internal