Merge 157273 "Fix threading races on HRTFElevation::audioBusMap"

Source/core/platform/audio/HRTFElevation.cpp

Index: Source/core/platform/audio/HRTFElevation.cpp
===================================================================
--- Source/core/platform/audio/HRTFElevation.cpp	(revision 157690)
+++ Source/core/platform/audio/HRTFElevation.cpp	(working copy)
@@ -37,6 +37,7 @@
 #include "core/platform/audio/AudioBus.h"
 #include "core/platform/audio/HRTFPanner.h"
 #include "wtf/OwnPtr.h"
+#include "wtf/ThreadingPrimitives.h"
 
 using namespace std;
 
@@ -64,7 +65,9 @@
 {
     typedef HashMap<String, RefPtr<AudioBus> > AudioBusMap;
     DEFINE_STATIC_LOCAL(AudioBusMap, audioBusMap, ());
+    DEFINE_STATIC_LOCAL(Mutex, mutex, ());
 
+    MutexLocker locker(mutex);
     RefPtr<AudioBus> bus;
     AudioBusMap::iterator iterator = audioBusMap.find(subjectName);
     if (iterator == audioBusMap.end()) {