Add missing includes checks.h/array_view.h

instead of relying on optional.h to included these 2 headers.

Bug: webrtc:9078
Change-Id: I7a4b3facd81690b8f107640487e129986c1f5ff6
Reviewed-on: https://webrtc-review.googlesource.com/68602
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#22803}

api/audio_codecs/audio_format.h

@@ -17,6 +17,7 @@
 #include <utility>
 
 #include "api/optional.h"
+#include "rtc_base/checks.h"
 
 namespace webrtc {