Fix DiscardableMemoryPool::free race condition

src/lazy/SkDiscardableMemoryPool.cpp

 /*
  * Copyright 2013 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "SkDiscardableMemory.h"
 #include "SkDiscardableMemoryPool.h"
 #include "SkImageGenerator.h"
@@ skipping 170 matching lines @@
     PoolDiscardableMemory* dm = SkNEW_ARGS(PoolDiscardableMemory,
                                              (this, addr, bytes));
     SkAutoMutexAcquire autoMutexAcquire(fMutex);
     fList.addToHead(dm);
     fUsed += bytes;
     this->dumpDownTo(fBudget);
     return dm;
 }
 
 void DiscardableMemoryPool::free(PoolDiscardableMemory* dm) {
+    SkAutoMutexAcquire autoMutexAcquire(fMutex);
     // This is called by dm's destructor.
     if (dm->fPointer != NULL) {
-        SkAutoMutexAcquire autoMutexAcquire(fMutex);
         sk_free(dm->fPointer);
         dm->fPointer = NULL;
         SkASSERT(fUsed >= dm->fBytes);
         fUsed -= dm->fBytes;
         fList.remove(dm);
     } else {
         SkASSERT(!fList.isInList(dm));
     }
 }
 
@@ skipping 56 matching lines @@
     return SkNEW_ARGS(DiscardableMemoryPool, (size, mutex));
 }
 
 SK_DECLARE_STATIC_LAZY_PTR(SkDiscardableMemoryPool, global, create_global_pool);
 
 SkDiscardableMemoryPool* SkGetGlobalDiscardableMemoryPool() {
     return global.get();
 }
 
 ////////////////////////////////////////////////////////////////////////////////