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/BUILD.gn b/rtc_base/BUILD.gn
index bcca552..effe58b 100644
--- a/rtc_base/BUILD.gn
+++ b/rtc_base/BUILD.gn
@@ -446,7 +446,7 @@
   deps += [
     "..:webrtc_common",
     "../api:array_view",
-    "../api:optional",
+    "//third_party/abseil-cpp/absl/types:optional",
   ]
 
   if (is_android) {
@@ -661,7 +661,7 @@
     ":checks",
     ":rtc_base_approved",
     ":safe_compare",
-    "../api:optional",
+    "//third_party/abseil-cpp/absl/types:optional",
   ]
 }
 
@@ -728,7 +728,7 @@
     ":stringutils",
     "..:webrtc_common",
     "../api:array_view",
-    "../api:optional",
+    "//third_party/abseil-cpp/absl/types:optional",
   ]
   public_deps = [
     ":rtc_base_approved",
@@ -1304,9 +1304,9 @@
       ":rtc_base_tests_utils",
       ":stringutils",
       "../api:array_view",
-      "../api:optional",
       "../test:fileutils",
       "../test:test_support",
+      "//third_party/abseil-cpp/absl/types:optional",
     ]
     public_deps = [
       ":rtc_base",