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..81ae45dba1cad49b503cfbfe5346856961b74680 100644
--- a/include/gpu/GrContext.h
+++ b/include/gpu/GrContext.h
@@ -390,6 +390,9 @@ private:
     SkMutex                         fReadPixelsMutex;
     SkMutex                         fTestPMConversionsMutex;
 
+    // In debug builds we take a debug mutex to guard against improper thread handling
+    SkDEBUGCODE(mutable SkRecursiveMutex fSafetyMutex;)
+
     struct CleanUpData {
         PFCleanUpFunc fFunc;
         void*         fInfo;