[Editing][Codehealth] Replace Position::uptream/downstream to

mostBackward/ForwardCaretPosition

Position::upstream calls just VisibleUnit.cpp::mostForwardCaretPosition and also 
downstream:
https://code.google.com/p/chromium/codesearch#chromium/src/third_party/WebKit/Source/c
ore/editing/Position.cpp&l=484&cl=GROK

This CL replaces it.
We will replace all upstream/downstream and remove them.

BUG=518738

Review URL: https://codereview.chromium.org/1306743002

git-svn-id: svn://svn.chromium.org/blink/trunk@200977 bbb929c8-8fbe-4397-9dbb-9b2b20218538

third_party/WebKit/Source/core/layout/HitTestResult.cpp

@@ -163,7 +163,7 @@ PositionWithAffinity HitTestResult::position() const
     if (!layoutObject)
         return PositionWithAffinity();
     if (m_innerPossiblyPseudoNode->isPseudoElement() && m_innerPossiblyPseudoNode->pseudoId() == BEFORE)
-        return Position(m_innerNode, PositionAnchorType::BeforeChildren).downstream();
+        return mostBackwardCaretPosition(Position(m_innerNode, PositionAnchorType::BeforeChildren));
     return layoutObject->positionForPoint(localPoint());
 }
 

third_party/WebKit/Source/core/layout/LayoutObject.cpp

@@ -2992,10 +2992,10 @@ PositionWithAffinity LayoutObject::createPositionWithAffinity(int offset, TextAf
         if (!node->hasEditableStyle()) {
             // If it can be found, we prefer a visually equivalent position that is editable.
             const Position position = Position(node, offset);
-            Position candidate = position.downstream(CanCrossEditingBoundary);
+            Position candidate = mostBackwardCaretPosition(position, CanCrossEditingBoundary);
             if (candidate.anchorNode()->hasEditableStyle())
                 return PositionWithAffinity(candidate, affinity);
-            candidate = position.upstream(CanCrossEditingBoundary);
+            candidate = mostForwardCaretPosition(position, CanCrossEditingBoundary);
             if (candidate.anchorNode()->hasEditableStyle())
                 return PositionWithAffinity(candidate, affinity);
         }

third_party/WebKit/Source/core/layout/LayoutView.cpp

@@ -803,11 +803,11 @@ void LayoutView::commitPendingSelectionAlgorithm()
     // as the start of the selection, the selection painting code will think that content on the line containing 'foo' is selected
     // and will fill the gap before 'bar'.
     PositionType startPos = Strategy::selectionStart(selection);
-    PositionType candidate = startPos.downstream();
+    PositionType candidate = mostBackwardCaretPosition(startPos);
     if (isVisuallyEquivalentCandidate(candidate))
         startPos = candidate;
     PositionType endPos = Strategy::selectionEnd(selection);
-    candidate = endPos.upstream();
+    candidate = mostForwardCaretPosition(endPos);
     if (isVisuallyEquivalentCandidate(candidate))
         endPos = candidate;