Add MemoryPurgeController

Source/core/layout/LayoutView.h

Index: Source/core/layout/LayoutView.h
diff --git a/Source/core/layout/LayoutView.h b/Source/core/layout/LayoutView.h
index 529c46a31e4e053289d9dc4bdf09272df60326ee..2d6d401e8f4b339dd1fd4ec6e61c7e8a5e4ac79d 100644
--- a/Source/core/layout/LayoutView.h
+++ b/Source/core/layout/LayoutView.h
@@ -32,6 +32,7 @@
 #include "core/layout/LayoutState.h"
 #include "core/layout/PaintInvalidationState.h"
 #include "core/layout/PendingSelection.h"
+#include "platform/MemoryPurgeController.h"
 #include "platform/PODFreeListArena.h"
 #include "platform/RuntimeEnabledFeatures.h"
 #include "platform/heap/Handle.h"
@@ -47,7 +48,9 @@ class LayoutQuote;
 // It's dimensions match that of the logical viewport (which may be different from
 // the visible viewport in fixed-layout mode), and it is always at position (0,0)
 // relative to the document (and so isn't necessarily in view).
-class CORE_EXPORT LayoutView final : public LayoutBlockFlow {
+// TODO(bashi): Remove MemoryPurgeEventListener. This just demonstrate how
+// MemoryPurgeController works.
+class CORE_EXPORT LayoutView final : public MemoryPurgeEventListener, public LayoutBlockFlow {
 public:
     explicit LayoutView(Document*);
     ~LayoutView() override;
@@ -192,6 +195,8 @@ public:
     void setCompositedDisplayList(PassOwnPtr<const CompositedDisplayList> compositedDisplayList) { m_compositedDisplayList = compositedDisplayList; }
     const CompositedDisplayList* compositedDisplayList() const { return m_compositedDisplayList.get(); }
 
+    void purgeMemory(MemoryPurgeMode, DeviceKind) override;
+
 private:
     void mapLocalToContainer(const LayoutBoxModelObject* paintInvalidationContainer, TransformState&, MapCoordinatesFlags = ApplyContainerFlip, bool* wasFixed = nullptr, const PaintInvalidationState* = nullptr) const override;