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/api/audio_codecs/g711/BUILD.gn b/webrtc/api/audio_codecs/g711/BUILD.gn
index fbf9248..aa86490 100644
--- a/webrtc/api/audio_codecs/g711/BUILD.gn
+++ b/webrtc/api/audio_codecs/g711/BUILD.gn
@@ -19,6 +19,7 @@
   ]
   deps = [
     "..:audio_codecs_api",
+    "../..:optional",
     "../../..:webrtc_common",
     "../../../modules/audio_coding:g711",
     "../../../rtc_base:rtc_base_approved",
@@ -32,6 +33,7 @@
   ]
   deps = [
     "..:audio_codecs_api",
+    "../..:optional",
     "../../..:webrtc_common",
     "../../../modules/audio_coding:g711",
     "../../../rtc_base:rtc_base_approved",
diff --git a/webrtc/api/audio_codecs/g711/audio_decoder_g711.h b/webrtc/api/audio_codecs/g711/audio_decoder_g711.h
index b9650c9..2824c32 100644
--- a/webrtc/api/audio_codecs/g711/audio_decoder_g711.h
+++ b/webrtc/api/audio_codecs/g711/audio_decoder_g711.h
@@ -16,7 +16,7 @@
 
 #include "webrtc/api/audio_codecs/audio_decoder.h"
 #include "webrtc/api/audio_codecs/audio_format.h"
-#include "webrtc/rtc_base/optional.h"
+#include "webrtc/api/optional.h"
 
 namespace webrtc {
 
diff --git a/webrtc/api/audio_codecs/g711/audio_encoder_g711.h b/webrtc/api/audio_codecs/g711/audio_encoder_g711.h
index a22650d..5750100 100644
--- a/webrtc/api/audio_codecs/g711/audio_encoder_g711.h
+++ b/webrtc/api/audio_codecs/g711/audio_encoder_g711.h
@@ -16,7 +16,7 @@
 
 #include "webrtc/api/audio_codecs/audio_encoder.h"
 #include "webrtc/api/audio_codecs/audio_format.h"
-#include "webrtc/rtc_base/optional.h"
+#include "webrtc/api/optional.h"
 
 namespace webrtc {