Fixing WebRTC after moving from src/webrtc to src/

In https://webrtc-review.googlesource.com/c/src/+/1560 we moved WebRTC
from src/webrtc to src/ (in order to preserve an healthy git history).
This CL takes care of fixing header guards, #include paths, etc...

NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
TBR=tommi@webrtc.org


Bug: chromium:611808
Change-Id: Iea91618212bee0af16aa3f05071eab8f93706578
Reviewed-on: https://webrtc-review.googlesource.com/1561
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Reviewed-by: Henrik Kjellander <kjellander@webrtc.org>
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#19846}
diff --git a/system_wrappers/source/metrics_default.cc b/system_wrappers/source/metrics_default.cc
index 6e5715e..3263ec0 100644
--- a/system_wrappers/source/metrics_default.cc
+++ b/system_wrappers/source/metrics_default.cc
@@ -7,13 +7,13 @@
 // be found in the AUTHORS file in the root of the source tree.
 //
 
-#include "webrtc/system_wrappers/include/metrics_default.h"
+#include "system_wrappers/include/metrics_default.h"
 
 #include <algorithm>
 
-#include "webrtc/rtc_base/criticalsection.h"
-#include "webrtc/rtc_base/thread_annotations.h"
-#include "webrtc/system_wrappers/include/metrics.h"
+#include "rtc_base/criticalsection.h"
+#include "rtc_base/thread_annotations.h"
+#include "system_wrappers/include/metrics.h"
 
 // Default implementation of histogram methods for WebRTC clients that do not
 // want to provide their own implementation.