Move paint code from RenderFlexibleBox to BlockPainter.

Source/core/paint/BlockPainter.h

Index: Source/core/paint/BlockPainter.h
diff --git a/Source/core/paint/BlockPainter.h b/Source/core/paint/BlockPainter.h
index 0364e7250e70013fc79280f04f9fef70d855615b..710f1a809e223ee50b3f882af8385b96b88aee81 100644
--- a/Source/core/paint/BlockPainter.h
+++ b/Source/core/paint/BlockPainter.h
@@ -11,6 +11,7 @@ namespace blink {
 
 struct PaintInfo;
 class LayoutPoint;
+class RenderFlexibleBox;
 
 class BlockPainter {
 public:
@@ -25,6 +26,7 @@ public:
     // (grid items, flex items) require this behavior as well, and this function exists as a helper for them.
     // It is expected that the caller will call this function independent of the value of paintInfo.phase.
     static void paintAsInlineBlock(RenderObject*, PaintInfo&, const LayoutPoint&);
+    static void paintChildrenOfFlexibleBox(RenderFlexibleBox&, PaintInfo&, const LayoutPoint& paintOffset);
 
 private:
     LayoutRect overflowRectForPaintRejection() const;