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/common_audio/signal_processing/downsample_fast_mips.c b/common_audio/signal_processing/downsample_fast_mips.c
index ac39401..0f3f3a0 100644
--- a/common_audio/signal_processing/downsample_fast_mips.c
+++ b/common_audio/signal_processing/downsample_fast_mips.c
@@ -8,7 +8,7 @@
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
-#include "webrtc/common_audio/signal_processing/include/signal_processing_library.h"
+#include "common_audio/signal_processing/include/signal_processing_library.h"
 
 // Version of WebRtcSpl_DownsampleFast() for MIPS platforms.
 int WebRtcSpl_DownsampleFast_mips(const int16_t* data_in,