Clean up includes and forward-declarations in the flowthread / multicol files.

Source/core/rendering/RenderFlowThread.h

Index: Source/core/rendering/RenderFlowThread.h
diff --git a/Source/core/rendering/RenderFlowThread.h b/Source/core/rendering/RenderFlowThread.h
index 74a6b9cb50c34fa38658469ba0c5b208af1935e2..042d6f727a510402165f04ca406321c8b0413930 100644
--- a/Source/core/rendering/RenderFlowThread.h
+++ b/Source/core/rendering/RenderFlowThread.h
@@ -30,17 +30,12 @@
 #ifndef RenderFlowThread_h
 #define RenderFlowThread_h
 
-
+#include "core/rendering/LayerFragment.h"
 #include "core/rendering/RenderBlockFlow.h"
-#include "wtf/HashCountedSet.h"
 #include "wtf/ListHashSet.h"
-#include "wtf/PassRefPtr.h"
 
 namespace blink {
 
-struct LayerFragment;
-typedef Vector<LayerFragment, 1> LayerFragments;
-class RenderFlowThread;
 class RenderMultiColumnSet;
 class RenderRegion;