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_statistics.h b/rtc_base/rate_statistics.h
index aa28529..fe5ae82 100644
--- a/rtc_base/rate_statistics.h
+++ b/rtc_base/rate_statistics.h
@@ -13,7 +13,7 @@
 
 #include <memory>
 
-#include "api/optional.h"
+#include "absl/types/optional.h"
 #include "typedefs.h"  // NOLINT(build/include)
 
 namespace webrtc {
@@ -42,7 +42,7 @@
   // from a monotonic clock. Ie, it doesn't matter if this call moves the
   // window, since any subsequent call to Update or Rate would still have moved
   // the window as much or more.
-  rtc::Optional<uint32_t> Rate(int64_t now_ms) const;
+  absl::optional<uint32_t> Rate(int64_t now_ms) const;
 
   // Update the size of the averaging window. The maximum allowed value for
   // window_size_ms is max_window_size_ms as supplied in the constructor.