Create debug only SkSingleOwner

include/gpu/GrContext.h

 /*
  * Copyright 2010 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef GrContext_DEFINED
 #define GrContext_DEFINED
 
@@ skipping 372 matching lines @@
     // from the sw backend. readSurfacePixels is reentrant in one case - when performing
     // the PM conversions test. To handle this we do the PM conversions test outside
     // of fReadPixelsMutex and use a separate mutex to guard it. When it re-enters
     // readSurfacePixels it will grab fReadPixelsMutex and release it before the outer
     // readSurfacePixels proceeds to grab it.
     // TODO: Stop pretending to make GrContext thread-safe for sw rasterization and provide
     // a mechanism to make a SkPicture safe for multithreaded sw rasterization.
     SkMutex                         fReadPixelsMutex;
     SkMutex                         fTestPMConversionsMutex;
 
+    // In debug builds we take a debug mutex to guard against improper thread handling

[mtklein, 2016/01/06 15:08:27]

I'd put it right here.

[joshualitt, 2016/01/06 15:09:47]

This will end up being used by a number of Gr* classes and SkGpuDevice.  I can
make it GrSingleOwner and move it to its own header.

+    SkDEBUGCODE(mutable SkSingleOwner fSingleOwner;)
+
     struct CleanUpData {
         PFCleanUpFunc fFunc;
         void*         fInfo;
     };
 
     SkTDArray<CleanUpData>          fCleanUpData;
 
     const uint32_t                  fUniqueID;
 
     SkAutoTDelete<GrDrawingManager> fDrawingManager;
@@ skipping 35 matching lines @@
     /**
      * A callback similar to the above for use by the TextBlobCache
      * TODO move textblob draw calls below context so we can use the call above.
      */
     static void TextBlobCacheOverBudgetCB(void* data);
 
     typedef SkRefCnt INHERITED;
 };
 
 #endif