Fix circular dependencies between optional, array_view, and rtc_base.

This splits things out of rtc_base and makes dependencies explicit.

Bug: webrtc:6828
Change-Id: Ib813c7bd9e4de7ab015acb917bc09ee7204ba7bd
Reviewed-on: https://webrtc-review.googlesource.com/31940
Commit-Queue: Patrik Höglund <phoglund@webrtc.org>
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#21245}

rtc_base/fileutils.h

@@ -13,13 +13,15 @@
 
 #include <string>
 
-#if !defined(WEBRTC_WIN)
+#if defined(WEBRTC_WIN)
+#include <windows.h>
+#else
 #include <dirent.h>
 #include <stdio.h>
 #include <sys/stat.h>
 #include <sys/types.h>
 #include <unistd.h>
-#endif
+#endif  // WEBRTC_WIN
 
 #include "rtc_base/checks.h"
 #include "rtc_base/constructormagic.h"