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

Source/core/paint/ScrollRecorder.cpp

Index: Source/core/paint/ScrollRecorder.cpp
diff --git a/Source/core/paint/ScrollRecorder.cpp b/Source/core/paint/ScrollRecorder.cpp
index bbc320b683e32d46d749edacfadbc2ce4a72fa05..7fedf69ab3338544b3681c36feaa2688c2bce67f 100644
--- a/Source/core/paint/ScrollRecorder.cpp
+++ b/Source/core/paint/ScrollRecorder.cpp
@@ -12,33 +12,50 @@
 
 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)
 {
+}
+
+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(BeginScrollDisplayItem::create(m_client, m_beginItemType, currentOffset));
+        m_context.displayItemList()->add(EndScrollDisplayItem::create(m_client, endItemType));
     } else {
-        BeginScrollDisplayItem scrollDisplayItem(m_client, m_beginItemType, currentOffset);
-        scrollDisplayItem.replay(m_context);
+        EndScrollDisplayItem endScrollDisplayItem(m_client, endItemType);
+        endScrollDisplayItem.replay(m_context);
     }
 }
 
-ScrollRecorder::~ScrollRecorder()
+void ScrollRecorder::begin(const IntSize& currentOffset)
 {
-    DisplayItem::Type endItemType = DisplayItem::scrollTypeToEndScrollType(m_beginItemType);
+    ASSERT(!m_engaged);
+    m_engaged = true;
+
     if (RuntimeEnabledFeatures::slimmingPaintEnabled()) {
         ASSERT(m_context.displayItemList());
         if (m_context.displayItemList()->displayItemConstructionIsDisabled())
             return;
-        m_context.displayItemList()->add(EndScrollDisplayItem::create(m_client, endItemType));
+        m_context.displayItemList()->add(BeginScrollDisplayItem::create(m_client, m_beginItemType, currentOffset));
     } else {
-        EndScrollDisplayItem endScrollDisplayItem(m_client, endItemType);
-        endScrollDisplayItem.replay(m_context);
+        BeginScrollDisplayItem scrollDisplayItem(m_client, m_beginItemType, currentOffset);
+        scrollDisplayItem.replay(m_context);
     }
 }