[Reland] Correct fixed-position recording for Slimming Paint

Source/core/paint/LayerFixedPositionRecorder.cpp

Index: Source/core/paint/LayerFixedPositionRecorder.cpp
diff --git a/Source/core/paint/LayerFixedPositionRecorder.cpp b/Source/core/paint/LayerFixedPositionRecorder.cpp
new file mode 100644
index 0000000000000000000000000000000000000000..966c99925587a33f5481c0efa860ab0e7d717692
--- /dev/null
+++ b/Source/core/paint/LayerFixedPositionRecorder.cpp
@@ -0,0 +1,54 @@
+// 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 "config.h"
+#include "core/paint/LayerFixedPositionRecorder.h"
+
+#include "core/layout/LayoutBoxModelObject.h"
+#include "platform/RuntimeEnabledFeatures.h"
+#include "platform/graphics/GraphicsContext.h"
+#include "platform/graphics/paint/DisplayItemList.h"
+#include "platform/graphics/paint/FixedPositionContainerDisplayItem.h"
+#include "platform/graphics/paint/FixedPositionDisplayItem.h"
+
+namespace blink {
+
+LayerFixedPositionRecorder::LayerFixedPositionRecorder(GraphicsContext& graphicsContext, const LayoutBoxModelObject& layoutObject)
+    : m_graphicsContext(graphicsContext)
+    , m_layoutObject(layoutObject)
+    , m_isFixedPosition(layoutObject.style()->position() == FixedPosition)
+    , m_isFixedPositionContainer(layoutObject.canContainFixedPositionObjects())
+{
+    if (!RuntimeEnabledFeatures::slimmingPaintCompositorLayerizationEnabled())
+        return;
+
+    if (m_graphicsContext.displayItemList()->displayItemConstructionIsDisabled())
+        return;
+
+    if (m_isFixedPosition)
+        m_graphicsContext.displayItemList()->add(BeginFixedPositionDisplayItem::create(m_layoutObject));
+
+    // TODO(trchen): Adding a pair of display items on every transformed
+    // element can be expensive. Investigate whether we can optimize out some
+    // of them if applicable.
+    if (m_isFixedPositionContainer)
+        m_graphicsContext.displayItemList()->add(BeginFixedPositionContainerDisplayItem::create(m_layoutObject));
+}
+
+LayerFixedPositionRecorder::~LayerFixedPositionRecorder()
+{
+    if (!RuntimeEnabledFeatures::slimmingPaintCompositorLayerizationEnabled())
+        return;
+
+    if (m_graphicsContext.displayItemList()->displayItemConstructionIsDisabled())
+        return;
+
+    if (m_isFixedPositionContainer)
+        m_graphicsContext.displayItemList()->add(EndFixedPositionContainerDisplayItem::create(m_layoutObject));
+
+    if (m_isFixedPosition)
+        m_graphicsContext.displayItemList()->add(EndFixedPositionDisplayItem::create(m_layoutObject));
+}
+
+} // namespace blink