[Editing][CodeHealth] Move Position::up/downstream implementation to VisibleUnit.cpp

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 3d010c5301a873cbfac3e3a5eaf8feae432ca97b..06546d62f20136cb4e69819d917b51ea4991ddff 100644
--- a/Source/core/editing/VisibleUnits.h
+++ b/Source/core/editing/VisibleUnits.h
@@ -43,6 +43,29 @@ class LocalFrame;
 
 enum EWordSide { RightWordIfOnBoundary = false, LeftWordIfOnBoundary = true };
 
+// Position
+// mostForward/BackwardCaretPosition are used for moving back and forth between
+// visually equivalent candidates.
+// For example, for the text node "foo     bar" where whitespace is
+// collapsible, there are two candidates that map to the VisiblePosition
+// between 'b' and the space, after first space and before last space.
+//
+// mostForwardCaretPosition returns the left candidate and also returs
+// [boundary, 0] for any of the positions from [boundary, 0] to the first
+// candidate in boundary, where
+// endsOfNodeAreVisuallyDistinctPositions(boundary) is true.
+//
+// mostBackwardCaretPosition() returns the right one and also returns the
+// last position in the last atomic node in boundary for all of the positions
+// in boundary after the last candidate, where
+// endsOfNodeAreVisuallyDistinctPositions(boundary).
+// FIXME: This function should never be called when the line box tree is dirty.
+// See https://bugs.webkit.org/show_bug.cgi?id=97264
+CORE_EXPORT Position mostForwardCaretPosition(const Position &, EditingBoundaryCrossingRule = CannotCrossEditingBoundary);
+CORE_EXPORT PositionInComposedTree mostForwardCaretPosition(const PositionInComposedTree &, EditingBoundaryCrossingRule = CannotCrossEditingBoundary);
+CORE_EXPORT Position mostBackwardCaretPosition(const Position &, EditingBoundaryCrossingRule = CannotCrossEditingBoundary);
+CORE_EXPORT PositionInComposedTree mostBackwardCaretPosition(const PositionInComposedTree &, EditingBoundaryCrossingRule = CannotCrossEditingBoundary);
+
 // words
 CORE_EXPORT VisiblePosition startOfWord(const VisiblePosition &, EWordSide = RightWordIfOnBoundary);
 CORE_EXPORT VisiblePosition endOfWord(const VisiblePosition &, EWordSide = RightWordIfOnBoundary);