Fix threading races on HRTFElevation::audioBusMap

Source/core/platform/audio/HRTFElevation.cpp

Index: Source/core/platform/audio/HRTFElevation.cpp
diff --git a/Source/core/platform/audio/HRTFElevation.cpp b/Source/core/platform/audio/HRTFElevation.cpp
index a37e696f50ed6ec52fd056bb615d41d7667a572b..a5c606cdb95535ce11d81a107c0da00fe832fcc6 100644
--- a/Source/core/platform/audio/HRTFElevation.cpp
+++ b/Source/core/platform/audio/HRTFElevation.cpp
@@ -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 @@ static PassRefPtr<AudioBus> getConcatenatedImpulseResponsesForSubject(const Stri
 {
     typedef HashMap<String, RefPtr<AudioBus> > AudioBusMap;
     DEFINE_STATIC_LOCAL(AudioBusMap, audioBusMap, ());
+    DEFINE_STATIC_LOCAL(Mutex, mutex, ());

[Ken Russell (switch to Gerrit), 2013/09/04 19:57:55]

This doesn't look any safer to me. If there is really a race then two threads
could plausibly call this function the first time and race for the
initialization of these static locals.

 
+    MutexLocker locker(mutex);
     RefPtr<AudioBus> bus;
     AudioBusMap::iterator iterator = audioBusMap.find(subjectName);
     if (iterator == audioBusMap.end()) {