Create debug only SkSingleOwner

include/gpu/GrDrawContext.h

 /*
  * Copyright 2015 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef GrDrawContext_DEFINED
 #define GrDrawContext_DEFINED
 
 #include "GrColor.h"
 #include "GrRenderTarget.h"
 #include "SkRefCnt.h"
 #include "SkSurfaceProps.h"
+#include "../private/GrSingleOwner.h"
 
 class GrClip;
 class GrContext;
 class GrDrawBatch;
 class GrDrawPathBatchBase;
 class GrDrawingManager;
 class GrDrawTarget;
 class GrPaint;
 class GrPathProcessor;
 class GrPipelineBuilder;
@@ skipping 274 matching lines @@
 
     GrDrawingManager* fDrawingManager;
     GrRenderTarget*   fRenderTarget;
 
     // In MDB-mode the drawTarget can be closed by some other drawContext that has picked
     // it up. For this reason, the drawTarget should only ever be accessed via 'getDrawTarget'.
     GrDrawTarget*     fDrawTarget;
     GrTextContext*    fTextContext; // lazily gotten from GrContext::DrawingManager
 
     SkSurfaceProps    fSurfaceProps;
+
+    // In debug builds we guard against improper thread handling
+    SkDEBUGCODE(mutable GrSingleOwner fSingleOwner;)
 };
 
 #endif