discardable shared memory: Allow resetting the allocator instance.

base/memory/discardable_memory_allocator.cc

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "base/memory/discardable_memory_allocator.h"
 
 #include "base/logging.h"
 
 namespace base {
 namespace {
 
 DiscardableMemoryAllocator* g_allocator = nullptr;
 
 }  // namespace
 
 // static
 void DiscardableMemoryAllocator::SetInstance(
     DiscardableMemoryAllocator* allocator) {
-  DCHECK(allocator);
-
-  // Make sure this function is only called once before the first call
-  // to GetInstance().
-  DCHECK(!g_allocator);
-
+  DCHECK(!allocator || !g_allocator);
   g_allocator = allocator;
 }
 
 // static
 DiscardableMemoryAllocator* DiscardableMemoryAllocator::GetInstance() {
   DCHECK(g_allocator);
   return g_allocator;
 }
 
 }  // namespace base