Add MemoryPurgeController

Source/platform/MemoryPurgeController.h

+// 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.
+
+#ifndef MemoryPurgeController_h
+#define MemoryPurgeController_h
+
+#include "platform/PlatformExport.h"
+#include "platform/heap/Handle.h"
+#include "wtf/MainThread.h"
+
+namespace blink {
+
+enum class MemoryPurgeMode {
+    // The tab contains the webview went to background
+    InactiveTab,
+    // TODO(bashi): Add more modes as needed.
+};
+
+enum class DeviceKind {
+    NotSpecified,
+    LowEnd,
+};
+
+// Classes which have discardable/reducible memory can implement this
+// interface to be informed when they should reduce memory consumption.
+// MemoryPurgeController assumes that subclasses of MemoryPurgeClient are
+// WillBes.
+class MemoryPurgeClient : public WillBeGarbageCollectedMixin {
+public:
+    virtual ~MemoryPurgeClient() { }
+
+    // MemoryPurgeController invokes this callback when a memory purge event
+    // has occurred.
+    virtual void purgeMemory(MemoryPurgeMode, DeviceKind) = 0;
+
+    DECLARE_TRACE();
+};
+
+// MemoryPurgeController listens to some events which could be opportunities
+// for reducing memory consumption and notifies its clients.
+// Since we want to control memory per tab, MemoryPurgeController is owned by
+// Page.
+class PLATFORM_EXPORT MemoryPurgeController {
+public:
+    MemoryPurgeController();
+
+    void registerClient(MemoryPurgeClient* client)
+    {
+        ASSERT(isMainThread());
+        ASSERT(client);
+        ASSERT(!m_clients.contains(client));
+        m_clients.add(client);
+    }
+
+    void unregisterClient(MemoryPurgeClient* client)
+    {
+        ASSERT(isMainThread());
+        ASSERT(m_clients.contains(client));
+        m_clients.remove(client);
+    }
+
+    DECLARE_TRACE();
+
+private:
+    void purgeMemory(MemoryPurgeMode);
+
+    WillBeHeapHashSet<RawPtrWillBeWeakMember<MemoryPurgeClient>> m_clients;
+    DeviceKind m_deviceKind;
+};
+
+} // namespace blink
+
+#endif // MemoryPurgeController_h