Make Selection#{base,extent}{Node,Offset} as aliases of {anchor,focus}{Node,Offset}

This patch makes |Selection#{base,extent}{Node,Offset}| as aliases of
|Selection#{anchor,focus}{Node,Offset}| instead of getting position from
|VisibleSelection| which we don't want to expose to JavaScript.

Note: Selection#{base,extent}{Node,Offset} are not standard API and there
are some discussion for standardizing them[1].

[1] https://github.com/w3c/selection-api/issues/34
Specify baseNode/baseOffset/extentNode/extentOffset?

Bug: 701501
Change-Id: I34fcdb4e80d5cef49bc1e0bdaea3cdec5c8b3758
Reviewed-on: https://chromium-review.googlesource.com/965663Reviewed-by: Yoichi Osato <yoichio@chromium.org>
Reviewed-by: Xiaocheng Hu <xiaochengh@chromium.org>
Commit-Queue: Yoshifumi Inoue <yosin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#543982}

third_party/WebKit/Source/core/editing/DOMSelection.cpp

@@ -129,14 +129,6 @@ static Position FocusPosition(const VisibleSelection& selection) {
   return focus.ParentAnchoredEquivalent();
 }
 
-static Position BasePosition(const VisibleSelection& selection) {
-  return selection.Base().ParentAnchoredEquivalent();
-}
-
-static Position ExtentPosition(const VisibleSelection& selection) {
-  return selection.Extent().ParentAnchoredEquivalent();
-}
-
 Node* DOMSelection::anchorNode() const {
   if (Range* range = PrimaryRangeOrNull()) {
     if (!GetFrame() || IsBaseFirstInSelection())
@@ -174,31 +166,19 @@ unsigned DOMSelection::focusOffset() const {
 }
 
 Node* DOMSelection::baseNode() const {
-  if (!IsAvailable())
-    return nullptr;
-
-  return ShadowAdjustedNode(BasePosition(GetVisibleSelection()));
+  return anchorNode();
 }
 
 unsigned DOMSelection::baseOffset() const {
-  if (!IsAvailable())
-    return 0;
-
-  return ShadowAdjustedOffset(BasePosition(GetVisibleSelection()));
+  return anchorOffset();
 }
 
 Node* DOMSelection::extentNode() const {
-  if (!IsAvailable())
-    return nullptr;
-
-  return ShadowAdjustedNode(ExtentPosition(GetVisibleSelection()));
+  return focusNode();
 }
 
 unsigned DOMSelection::extentOffset() const {
-  if (!IsAvailable())
-    return 0;
-
-  return ShadowAdjustedOffset(ExtentPosition(GetVisibleSelection()));
+  return focusOffset();
 }
 
 bool DOMSelection::isCollapsed() const {