Allow certain SP recorder classes to defer their "begin" operation.

Source/core/paint/ScrollRecorder.cpp

 // 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/ScrollRecorder.h"
 
 #include "platform/RuntimeEnabledFeatures.h"
 #include "platform/graphics/GraphicsContext.h"
 #include "platform/graphics/paint/DisplayItemList.h"
 #include "platform/graphics/paint/ScrollDisplayItem.h"
 
 namespace blink {
 
-ScrollRecorder::ScrollRecorder(GraphicsContext& context, const DisplayItemClientWrapper& client, PaintPhase phase, const IntSize& currentOffset)
+ScrollRecorder::ScrollRecorder(GraphicsContext& context, const DisplayItemClientWrapper& client, PaintPhase phase)
     : m_client(client)
     , m_beginItemType(DisplayItem::paintPhaseToScrollType(phase))
     , m_context(context)
+    , m_engaged(false)
 {
-    if (RuntimeEnabledFeatures::slimmingPaintEnabled()) {
-        ASSERT(m_context.displayItemList());
-        if (m_context.displayItemList()->displayItemConstructionIsDisabled())
-            return;
-        m_context.displayItemList()->add(BeginScrollDisplayItem::create(m_client, m_beginItemType, currentOffset));
-    } else {
-        BeginScrollDisplayItem scrollDisplayItem(m_client, m_beginItemType, currentOffset);
-        scrollDisplayItem.replay(m_context);
-    }
+}
+
+ScrollRecorder::ScrollRecorder(GraphicsContext& context, const DisplayItemClientWrapper& client, PaintPhase phase, const IntSize& currentOffset)
+    : ScrollRecorder(context, client, phase)
+{
+    begin(currentOffset);
 }
 
 ScrollRecorder::~ScrollRecorder()
 {
+    if (!m_engaged)
+        return;
+
     DisplayItem::Type endItemType = DisplayItem::scrollTypeToEndScrollType(m_beginItemType);
     if (RuntimeEnabledFeatures::slimmingPaintEnabled()) {
         ASSERT(m_context.displayItemList());
         if (m_context.displayItemList()->displayItemConstructionIsDisabled())
             return;
         m_context.displayItemList()->add(EndScrollDisplayItem::create(m_client, endItemType));
     } else {
         EndScrollDisplayItem endScrollDisplayItem(m_client, endItemType);
         endScrollDisplayItem.replay(m_context);
     }
 }
 
+void ScrollRecorder::begin(const IntSize& currentOffset)
+{
+    ASSERT(!m_engaged);
+    m_engaged = true;
+
+    if (RuntimeEnabledFeatures::slimmingPaintEnabled()) {
+        ASSERT(m_context.displayItemList());
+        if (m_context.displayItemList()->displayItemConstructionIsDisabled())
+            return;
+        m_context.displayItemList()->add(BeginScrollDisplayItem::create(m_client, m_beginItemType, currentOffset));
+    } else {
+        BeginScrollDisplayItem scrollDisplayItem(m_client, m_beginItemType, currentOffset);
+        scrollDisplayItem.replay(m_context);
+    }
+}
+
 } // namespace blink