discardable shared memory: Allow resetting the allocator instance.

ui/views/mus/mus_client.cc

 // Copyright 2016 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 "ui/views/mus/mus_client.h"
 
 #include "base/bind.h"
 #include "base/memory/ptr_util.h"
 #include "base/threading/thread.h"
 #include "components/discardable_memory/client/client_discardable_shared_memory_manager.h"
@@ skipping 110 matching lines @@
   window_tree_client_.reset();
   ui::OSExchangeDataProviderFactory::SetFactory(nullptr);
   ui::Clipboard::DestroyClipboardForCurrentThread();
   gpu_.reset();
 
   if (ViewsDelegate::GetInstance()) {
     ViewsDelegate::GetInstance()->set_native_widget_factory(
         ViewsDelegate::NativeWidgetFactory());
   }
 
+  base::DiscardableMemoryAllocator::SetInstance(nullptr);
   DCHECK_EQ(instance_, this);
   instance_ = nullptr;
 }
 
 // static
 bool MusClient::ShouldCreateDesktopNativeWidgetAura(
     const Widget::InitParams& init_params) {
   // TYPE_CONTROL and child widgets require a NativeWidgetAura.
   return init_params.type != Widget::InitParams::TYPE_CONTROL &&
          !init_params.child;
@@ skipping 134 matching lines @@
       return root->GetTopWindowContainingPoint(relative_point);
   }
   return nullptr;
 }
 
 std::unique_ptr<OSExchangeData::Provider> MusClient::BuildProvider() {
   return base::MakeUnique<aura::OSExchangeDataProviderMus>();
 }
 
 }  // namespace views