Add additional assert to GrSingleOwner

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 19 matching lines @@
      void enter() {
          SkAutoMutexAcquire lock(fMutex);
          SkThreadID self = SkGetThreadID();
          SkASSERT(fOwner == self || fOwner == kIllegalThreadID);
          fReentranceCount++;
          fOwner = self;
      }
 
      void exit() {
          SkAutoMutexAcquire lock(fMutex);
+         SkASSERT(fOwner == SkGetThreadID());
          fReentranceCount--;
          if (fReentranceCount == 0) {
              fOwner = kIllegalThreadID;
          }
      }
 
      SkMutex fMutex;
      SkThreadID fOwner;    // guarded by fMutex
      int fReentranceCount; // guarded by fMutex
 };
 #endif
 
 #endif