Revert "Remove WEBRTC_TRACE."

This reverts commit 2209b90449473e1df3e0797b6271c7624b41907d.

Reason for revert: breaks Chromium

Original change's description:
> Remove WEBRTC_TRACE.
> 
> Bug: webrtc:5118
> Change-Id: I0af0f8845ee016fa61d7cecc526e2a672ec8732d
> Reviewed-on: https://webrtc-review.googlesource.com/5382
> Reviewed-by: Niels Moller <nisse@webrtc.org>
> Reviewed-by: Sami Kalliomäki <sakal@webrtc.org>
> Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
> Commit-Queue: Fredrik Solenberg <solenberg@webrtc.org>
> Cr-Commit-Position: refs/heads/master@{#20114}

TBR=solenberg@webrtc.org,sakal@webrtc.org,kwiberg@webrtc.org,nisse@webrtc.org

Change-Id: Ie54fc05c1d7895c088cba410ed87a7c9a0701427
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: webrtc:5118
Reviewed-on: https://webrtc-review.googlesource.com/5980
Reviewed-by: Fredrik Solenberg <solenberg@webrtc.org>
Commit-Queue: Fredrik Solenberg <solenberg@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#20115}
diff --git a/modules/audio_processing/audio_processing_unittest.cc b/modules/audio_processing/audio_processing_unittest.cc
index 6893159..7cd2c95 100644
--- a/modules/audio_processing/audio_processing_unittest.cc
+++ b/modules/audio_processing/audio_processing_unittest.cc
@@ -38,6 +38,7 @@
 #include "rtc_base/task_queue.h"
 #include "rtc_base/thread.h"
 #include "system_wrappers/include/event_wrapper.h"
+#include "system_wrappers/include/trace.h"
 #include "test/gtest.h"
 #include "test/testsupport/fileutils.h"
 
@@ -350,9 +351,11 @@
   virtual void TearDown();
 
   static void SetUpTestCase() {
+    Trace::CreateTrace();
   }
 
   static void TearDownTestCase() {
+    Trace::ReturnTrace();
     ClearTempFiles();
   }