Avoid re-iterate out-of-order display items

Source/platform/graphics/paint/DisplayItem.h

Index: Source/platform/graphics/paint/DisplayItem.h
diff --git a/Source/platform/graphics/paint/DisplayItem.h b/Source/platform/graphics/paint/DisplayItem.h
index 55f0db17720c57a732e940235ab47d5a6b2e1887..31f9691d2e240e31e1276c7a3dcca320e75c62ee 100644
--- a/Source/platform/graphics/paint/DisplayItem.h
+++ b/Source/platform/graphics/paint/DisplayItem.h
@@ -184,7 +184,6 @@ public:
         , m_type(type)
         , m_derivedSize(derivedSize)
         , m_skippedCache(false)
-        , m_ignoredFromList(false)
 #ifndef NDEBUG
         , m_clientDebugString(client.debugName())
 #endif
@@ -195,6 +194,8 @@ public:
         ASSERT_WITH_SECURITY_IMPLICATION(derivedSize >= sizeof(*this));
     }
 
+    virtual ~DisplayItem() { }
+
     // Ids are for matching new DisplayItems with existing DisplayItems.
     struct Id {
         Id(const DisplayItemClient client, const Type type, const unsigned scope)
@@ -233,8 +234,6 @@ public:
         return Id(m_client, nonCachedType(), m_scope);
     }
 
-    virtual ~DisplayItem() { }
-
     virtual void replay(GraphicsContext&) { }
 
     DisplayItemClient client() const { return m_client; }
@@ -329,8 +328,13 @@ public:
 
     virtual bool drawsContent() const { return false; }
 
-    bool ignoreFromDisplayList() const { return m_ignoredFromList; }
-    void setIgnoredFromDisplayList() { m_ignoredFromList = true; }
+    bool isValid() const { return m_client; }
+    // Re-construct this display item as a permanently invalid item with no client set.
+    void invalidate()

[pdr., 2015/08/18 06:15:21]

This looks correct now, but it's tricky. Because this codepath is only used for
underinvalidation checking, what do you think about renaming invalidate() to
clearClientForUnderinvalidationChecking(), changing m_client to be mutable, and
directly setting m_client to nullptr?

[Xianzhu, 2015/08/18 16:24:44]

Acknowledged and Done.

+    {
+        this->~DisplayItem();
+        new (this) DisplayItem;
+    }
 
 #ifndef NDEBUG
     static WTF::String typeAsDebugString(DisplayItem::Type);
@@ -351,7 +355,6 @@ private:
         , m_type(UninitializedType)
         , m_derivedSize(sizeof(*this))
         , m_skippedCache(false)
-        , m_ignoredFromList(true)
 #ifndef NDEBUG
         , m_clientDebugString("invalid")
 #endif
@@ -363,7 +366,6 @@ private:
     const Type m_type : 16;
     unsigned m_derivedSize : 8; // size of the actual derived class
     unsigned m_skippedCache : 1;
-    unsigned m_ignoredFromList : 1;
 
 #ifndef NDEBUG
     WTF::String m_clientDebugString;