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/modules/audio_processing/BUILD.gn b/webrtc/modules/audio_processing/BUILD.gn
index e0719b4..4887250 100644
--- a/webrtc/modules/audio_processing/BUILD.gn
+++ b/webrtc/modules/audio_processing/BUILD.gn
@@ -238,6 +238,7 @@
     "..:module_api",
     "../..:webrtc_common",
     "../../api:array_view",
+    "../../api:optional",
     "../../audio/utility:audio_frame_operations",
     "../../rtc_base:gtest_prod",
     "../../rtc_base:protobuf_utils",
@@ -553,6 +554,7 @@
       "..:module_api",
       "../..:webrtc_common",
       "../../api:array_view",
+      "../../api:optional",
       "../../common_audio:common_audio",
       "../../rtc_base:gtest_prod",
       "../../rtc_base:protobuf_utils",
@@ -747,6 +749,7 @@
         ":audioproc_debug_proto",
         ":audioproc_protobuf_utils",
         ":audioproc_test_utils",
+        "../../api:optional",
         "../../common_audio:common_audio",
         "../../rtc_base:protobuf_utils",
         "../../rtc_base:rtc_base_approved",
@@ -778,6 +781,7 @@
       ":audio_processing",
       "..:module_api",
       "../../api:array_view",
+      "../../api:optional",
       "../../common_audio",
       "../../rtc_base:rtc_base_approved",
       "../../system_wrappers:system_wrappers",