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/rate_limiter.cc b/rtc_base/rate_limiter.cc
index 34e0a40..0343f25 100644
--- a/rtc_base/rate_limiter.cc
+++ b/rtc_base/rate_limiter.cc
@@ -29,7 +29,7 @@
 bool RateLimiter::TryUseRate(size_t packet_size_bytes) {
   rtc::CritScope cs(&lock_);
   int64_t now_ms = clock_->TimeInMilliseconds();
-  rtc::Optional<uint32_t> current_rate = current_rate_.Rate(now_ms);
+  absl::optional<uint32_t> current_rate = current_rate_.Rate(now_ms);
   if (current_rate) {
     // If there is a current rate, check if adding bytes would cause maximum
     // bitrate target to be exceeded. If there is NOT a valid current rate,