Create debug only SkSingleOwner

include/gpu/GrContext.h

Index: include/gpu/GrContext.h
diff --git a/include/gpu/GrContext.h b/include/gpu/GrContext.h
index fdc6167b14f346358966e455292c2ffe37cc89e2..39477e0db81fbdc0875aec82f4220664336be11f 100644
--- a/include/gpu/GrContext.h
+++ b/include/gpu/GrContext.h
@@ -15,9 +15,10 @@
 #include "GrRenderTarget.h"
 #include "GrTextureProvider.h"
 #include "SkMatrix.h"
-#include "../private/SkMutex.h"
 #include "SkPathEffect.h"
 #include "SkTypes.h"
+#include "../private/GrSingleOwner.h"
+#include "../private/SkMutex.h"
 
 struct GrBatchAtlasConfig;
 class GrBatchFontCache;
@@ -390,6 +391,9 @@ private:
     SkMutex                         fReadPixelsMutex;
     SkMutex                         fTestPMConversionsMutex;
 
+    // In debug builds we guard against improper thread handling
+    SkDEBUGCODE(mutable GrSingleOwner fSingleOwner;)
+
     struct CleanUpData {
         PFCleanUpFunc fFunc;
         void*         fInfo;