Move optional.h to webrtc/api/

webrtc/modules/video_coding/BUILD.gn

Index: webrtc/modules/video_coding/BUILD.gn
diff --git a/webrtc/modules/video_coding/BUILD.gn b/webrtc/modules/video_coding/BUILD.gn
index 3af09ea23285c135b369ccc371d89d69b2a4dbaf..3bf221be17009d5bebdd3cf9d47dbffbe4e6acb3 100644
--- a/webrtc/modules/video_coding/BUILD.gn
+++ b/webrtc/modules/video_coding/BUILD.gn
@@ -96,6 +96,7 @@ rtc_static_library("video_coding") {
     ":webrtc_vp9",
     "..:module_api",
     "../..:webrtc_common",
+    "../../api:optional",
     "../../call:video_stream_api",
     "../../common_video",
     "../../rtc_base:rtc_base",
@@ -136,6 +137,7 @@ rtc_static_library("video_coding_utility") {
   deps = [
     "..:module_api",
     "../..:webrtc_common",
+    "../../api:optional",
     "../../api/video_codecs:video_codecs_api",
     "../../common_video",
     "../../modules/rtp_rtcp:rtp_rtcp",
@@ -232,6 +234,7 @@ rtc_static_library("webrtc_vp8") {
     ":video_coding_utility",
     "..:module_api",
     "../..:webrtc_common",
+    "../../api:optional",
     "../../api/video_codecs:video_codecs_api",
     "../../common_video",
     "../../rtc_base:rtc_base_approved",
@@ -438,6 +441,7 @@ if (rtc_include_tests) {
       ":webrtc_h264",
       ":webrtc_vp8",
       ":webrtc_vp9",
+      "../../api:optional",
       "../../api:video_frame_api",
       "../../common_video:common_video",
       "../../rtc_base:rtc_base_approved",