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/fuzzers/audio_decoder_fuzzer.cc b/webrtc/test/fuzzers/audio_decoder_fuzzer.cc
index ff4478d..441b12b 100644
--- a/webrtc/test/fuzzers/audio_decoder_fuzzer.cc
+++ b/webrtc/test/fuzzers/audio_decoder_fuzzer.cc
@@ -13,9 +13,9 @@
 #include <limits>
 
 #include "webrtc/api/audio_codecs/audio_decoder.h"
+#include "webrtc/api/optional.h"
 #include "webrtc/modules/rtp_rtcp/source/byte_io.h"
 #include "webrtc/rtc_base/checks.h"
-#include "webrtc/rtc_base/optional.h"
 
 namespace webrtc {
 namespace {