Refactor ResourceLoadPriorityOptimizer to avoid walking render tree

Source/core/rendering/RenderObject.h

Index: Source/core/rendering/RenderObject.h
diff --git a/Source/core/rendering/RenderObject.h b/Source/core/rendering/RenderObject.h
index 066ee73d80ecbaf37aa2742d1ed070f846b210ef..f8a5946b59291a90125433fed7b179902e195573 100644
--- a/Source/core/rendering/RenderObject.h
+++ b/Source/core/rendering/RenderObject.h
@@ -668,8 +668,9 @@ public:
     // Subclasses must reimplement this method to compute the size and position
     // of this object and all its descendants.
     virtual void layout() = 0;
-    virtual void didLayout(ResourceLoadPriorityOptimizer&);
-    virtual void didScroll(ResourceLoadPriorityOptimizer&);
+    virtual bool updateImageLoadingPriorities() { return false; }
+    void setHasPendingResourceUpdate(bool hasPendingResourceUpdate) { m_bitfields.setHasPendingResourceUpdate(hasPendingResourceUpdate); }
+    bool hasPendingResourceUpdate() const { return m_bitfields.hasPendingResourceUpdate(); }
 
     /* This function performs a layout only if one is needed. */
     void layoutIfNeeded() { if (needsLayout()) layout(); }
@@ -1147,10 +1148,11 @@ private:
             , m_selectionState(SelectionNone)
             , m_flowThreadState(NotInsideFlowThread)
             , m_boxDecorationState(NoBoxDecorations)
+            , m_hasPendingResourceUpdate(false)
         {
         }
 
-        // 32 bits have been used in the first word, and 2 in the second.
+        // 32 bits have been used in the first word, and 3 in the second.
         ADD_BOOLEAN_BITFIELD(selfNeedsLayout, SelfNeedsLayout);
         ADD_BOOLEAN_BITFIELD(shouldDoFullRepaintAfterLayout, ShouldDoFullRepaintAfterLayout);
         ADD_BOOLEAN_BITFIELD(shouldRepaintOverflowIfNeeded, ShouldRepaintOverflowIfNeeded);
@@ -1191,6 +1193,9 @@ private:
         unsigned m_boxDecorationState : 2; // BoxDecorationState
 
     public:
+
+        ADD_BOOLEAN_BITFIELD(hasPendingResourceUpdate, HasPendingResourceUpdate);
+
         bool isOutOfFlowPositioned() const { return m_positionedState == IsOutOfFlowPositioned; }
         bool isRelPositioned() const { return m_positionedState == IsRelativelyPositioned; }
         bool isStickyPositioned() const { return m_positionedState == IsStickyPositioned; }