Replace forward class declarations of VisiblePosition to include directives

Source/core/editing/VisibleUnits.h

Index: Source/core/editing/VisibleUnits.h
diff --git a/Source/core/editing/VisibleUnits.h b/Source/core/editing/VisibleUnits.h
index 0359b3363e41a1a87a29b0c9797ad01563de8c46..90ed23df55d4852aa7143e6e055300024582afc4 100644
--- a/Source/core/editing/VisibleUnits.h
+++ b/Source/core/editing/VisibleUnits.h
@@ -29,6 +29,7 @@
 #include "core/CoreExport.h"
 #include "core/editing/EditingBoundary.h"
 #include "core/editing/PositionWithAffinity.h"
+#include "core/editing/VisiblePosition.h"
 #include "platform/text/TextDirection.h"
 
 namespace blink {
@@ -37,7 +38,6 @@ class LayoutRect;
 class LayoutUnit;
 class LayoutObject;
 class Node;
-class VisiblePosition;
 class IntPoint;
 class InlineBox;
 class LocalFrame;