Make a single GrSingleOwner in GrContext

include/private/GrSingleOwner.h

 /*
  * Copyright 2016 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef GrSingleOwner_DEFINED
 #define GrSingleOwner_DEFINED
 
@@ skipping 29 matching lines @@
          fReentranceCount--;
          if (fReentranceCount == 0) {
              fOwner = kIllegalThreadID;
          }
      }
 
      SkMutex fMutex;
      SkThreadID fOwner;    // guarded by fMutex
      int fReentranceCount; // guarded by fMutex
 };
+#else
+class GrSingleOwner {}; // Provide a dummy implementation so we can pass pointers to constructors
 #endif
 
 #endif