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

This reverts commit https://codereview.webrtc.org/2963273002/
where the git cl format breaks include order on Windows.

BUG=webrtc:7634
NOTRY=True
TBR=kwiberg@webrtc.org

Review-Url: https://codereview.webrtc.org/2962303003 .
Cr-Commit-Position: refs/heads/master@{#18857}
diff --git a/webrtc/rtc_base/mathutils.h b/webrtc/rtc_base/mathutils.h
index e57697c..3882c32 100644
--- a/webrtc/rtc_base/mathutils.h
+++ b/webrtc/rtc_base/mathutils.h
@@ -14,7 +14,7 @@
 #include <math.h>
 #include <type_traits>
 
-#include "webrtc/rtc_base/checks.h"
+#include "webrtc/base/checks.h"
 
 #ifndef M_PI
 #define M_PI 3.14159265359f