Move optional.h to webrtc/api/

We use Optional in our public API, so its header should be in
webrtc/api/.

BUG=webrtc:8205

Review-Url: https://codereview.webrtc.org/3011943002
Cr-Commit-Position: refs/heads/master@{#19693}
diff --git a/webrtc/test/BUILD.gn b/webrtc/test/BUILD.gn
index 0858468..9ec0290 100644
--- a/webrtc/test/BUILD.gn
+++ b/webrtc/test/BUILD.gn
@@ -60,6 +60,7 @@
 
   deps = [
     "..:webrtc_common",
+    "../api:optional",
     "../api/video_codecs:video_codecs_api",
     "../call:video_stream_api",
     "../common_video",
@@ -375,6 +376,7 @@
   ]
   deps = [
     "..:webrtc_common",
+    "../api:optional",
     "../rtc_base:rtc_base_approved",
   ]
   if (is_ios) {
@@ -437,6 +439,7 @@
   deps = [
     ":fileutils",
     ":test_support",
+    "../api:optional",
     "../rtc_base:rtc_base_approved",
     "//testing/gmock",
     "//testing/gtest",