Revert "Update includes for webrtc/{base => rtc_base} rename (2/3)"

This reverts commit c3771cc4d37f5573fe53b7c7cff295a4f0f9560f.
(breaks downstream internal project)

BUG=webrtc:7634
NOTRY=True
NOPRESUBMIT=True

Review-Url: https://codereview.webrtc.org/2972463002 .
Cr-Commit-Position: refs/heads/master@{#18873}
diff --git a/webrtc/test/testsupport/fileutils.h b/webrtc/test/testsupport/fileutils.h
index 570d7b4..434287d 100644
--- a/webrtc/test/testsupport/fileutils.h
+++ b/webrtc/test/testsupport/fileutils.h
@@ -16,7 +16,7 @@
 #include <string>
 #include <vector>
 
-#include "webrtc/rtc_base/optional.h"
+#include "webrtc/base/optional.h"
 
 namespace webrtc {
 namespace test {