Prune ComputeVisibleSelectionInDOMTreeDeprecated from LayoutTreeAsText

This patch replaces a call site of the deprecated function by the
proper version (which requires clean layout), which is safe because
the layout is:
- Either updated by the call site (ExternalRepresentation)
- Or intentionally kept dirty due to kLayoutAsTextDontUpdateLayout flag

Bug: 698633
Change-Id: I09c3be97ac43e5327cb1175999c83454a15c40f1
Reviewed-on: https://chromium-review.googlesource.com/596709Reviewed-by: Koji Ishii <kojii@chromium.org>
Reviewed-by: Emil A Eklund <eae@chromium.org>
Commit-Queue: Xiaocheng Hu <xiaochengh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#491438}

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

@@ -776,8 +776,8 @@ static void WriteSelection(TextStream& ts, const LayoutObject* o) {
   if (!frame)
     return;
 
-  VisibleSelection selection =
-      frame->Selection().ComputeVisibleSelectionInDOMTreeDeprecated();
+  const VisibleSelection& selection =
+      frame->Selection().ComputeVisibleSelectionInDOMTree();
   if (selection.IsCaret()) {
     ts << "caret: position " << selection.Start().ComputeEditingOffset()
        << " of " << NodePosition(selection.Start().AnchorNode());