memory coordinator: Purge memory under memory pressure

content/public/browser/memory_coordinator_delegate.h

 // 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.
 
 #ifndef CONTENT_PUBLIC_BROWSER_MEMORY_COORDINATOR_DELEGATE_H_
 #define CONTENT_PUBLIC_BROWSER_MEMORY_COORDINATOR_DELEGATE_H_
 
 #include "content/common/content_export.h"
 
 namespace content {
 
 // A delegate class which is used by the memory coordinator.
 class CONTENT_EXPORT MemoryCoordinatorDelegate {
  public:
   virtual ~MemoryCoordinatorDelegate() {}
 
   // Returns true when a given backgrounded renderer process can be suspended.
   virtual bool CanSuspendBackgroundedRenderer(int render_process_id) = 0;
 
   // Requests to discard one tab. An implementation should select a low priority
   // tab to discard. If there is no tab that can be discarded, this doesn't take
-  // effect.
-  virtual void DiscardTab() {}
+  // effect and returns false.
+  virtual bool DiscardTab();
 };
 
 }  // namespace content
 
 #endif  // CONTENT_PUBLIC_BROWSER_MEMORY_COORDINATOR_DELEGATE_H_