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/numerics/sequence_number_util.h

@@ -14,7 +14,7 @@
 #include <limits>
 #include <type_traits>
 
-#include "api/optional.h"
+#include "absl/types/optional.h"
 #include "rtc_base/numerics/mod_ops.h"
 #include "rtc_base/numerics/safe_compare.h"
 
@@ -120,7 +120,7 @@ class SeqNumUnwrapper {
 
  private:
   uint64_t last_unwrapped_;
-  rtc::Optional<T> last_value_;
+  absl::optional<T> last_value_;
 };
 
 }  // namespace webrtc