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/string_to_number.h

@@ -14,7 +14,7 @@
 #include <limits>
 #include <string>
 
-#include "api/optional.h"
+#include "absl/types/optional.h"
 
 namespace rtc {
 
@@ -25,8 +25,8 @@ namespace rtc {
 // are disabled in WebRTC.
 //
 // Integers are parsed using one of the following functions:
-//   rtc::Optional<int-type> StringToNumber(const char* str, int base = 10);
-//   rtc::Optional<int-type> StringToNumber(const std::string& str,
+//   absl::optional<int-type> StringToNumber(const char* str, int base = 10);
+//   absl::optional<int-type> StringToNumber(const std::string& str,
 //                                          int base = 10);
 //
 // These functions parse a value from the beginning of a string into one of the
@@ -46,13 +46,13 @@ namespace string_to_number_internal {
 using unsigned_type = unsigned long long;  // NOLINT(runtime/int)
 using signed_type = long long;             // NOLINT(runtime/int)
 
-rtc::Optional<signed_type> ParseSigned(const char* str, int base);
-rtc::Optional<unsigned_type> ParseUnsigned(const char* str, int base);
+absl::optional<signed_type> ParseSigned(const char* str, int base);
+absl::optional<unsigned_type> ParseUnsigned(const char* str, int base);
 }  // namespace string_to_number_internal
 
 template <typename T>
 typename std::enable_if<std::is_integral<T>::value && std::is_signed<T>::value,
-                        rtc::Optional<T>>::type
+                        absl::optional<T>>::type
 StringToNumber(const char* str, int base = 10) {
   using string_to_number_internal::signed_type;
   static_assert(
@@ -61,31 +61,31 @@ StringToNumber(const char* str, int base = 10) {
           std::numeric_limits<T>::lowest() >=
               std::numeric_limits<signed_type>::lowest(),
       "StringToNumber only supports signed integers as large as long long int");
-  rtc::Optional<signed_type> value =
+  absl::optional<signed_type> value =
       string_to_number_internal::ParseSigned(str, base);
   if (value && *value >= std::numeric_limits<T>::lowest() &&
       *value <= std::numeric_limits<T>::max()) {
     return static_cast<T>(*value);
   }
-  return rtc::nullopt;
+  return absl::nullopt;
 }
 
 template <typename T>
 typename std::enable_if<std::is_integral<T>::value &&
                             std::is_unsigned<T>::value,
-                        rtc::Optional<T>>::type
+                        absl::optional<T>>::type
 StringToNumber(const char* str, int base = 10) {
   using string_to_number_internal::unsigned_type;
   static_assert(std::numeric_limits<T>::max() <=
                     std::numeric_limits<unsigned_type>::max(),
                 "StringToNumber only supports unsigned integers as large as "
                 "unsigned long long int");
-  rtc::Optional<unsigned_type> value =
+  absl::optional<unsigned_type> value =
       string_to_number_internal::ParseUnsigned(str, base);
   if (value && *value <= std::numeric_limits<T>::max()) {
     return static_cast<T>(*value);
   }
-  return rtc::nullopt;
+  return absl::nullopt;
 }
 
 // The std::string overloads only exists if there is a matching const char*