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}

rtc_base/string_to_number.cc

@@ -8,10 +8,12 @@
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
+#include "rtc_base/string_to_number.h"
+
 #include <cerrno>
 #include <cstdlib>
 
-#include "rtc_base/string_to_number.h"
+#include "rtc_base/checks.h"
 
 namespace rtc {
 namespace string_to_number_internal {