Re-factor Editor::unappliedEditing()

This patch changes |Editor::unappliedEditing()| to use |isNone()| predicate
rather than checking |Document| equality of |Position| to simplify source code
for improving code health.

Since |correctVisibleSelection()| always returns |VisibileSlection| which
is an empty or valid for specified |Document|, checking equality of |Document|
between selection start and |frame.document()| is equivalent to empty selection
check.

BUG=n/a
TEST=n/a; no behavior changes

Review-Url: https://codereview.chromium.org/2540613002
Cr-Commit-Position: refs/heads/master@{#434961}

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

@@ -875,13 +875,14 @@ void Editor::unappliedEditing(EditCommandComposition* cmd) {
   // VisibleSelections as starting and ending selections.
   frame().document()->updateStyleAndLayoutIgnorePendingStylesheets();
 
-  VisibleSelection newSelection =
+  const VisibleSelection& newSelection =
       correctedVisibleSelection(cmd->startingSelection());
-  if (newSelection.start().document() == frame().document() &&
-      newSelection.end().document() == frame().document())
+  DCHECK(newSelection.isValidFor(*frame().document())) << newSelection;
+  if (!newSelection.isNone()) {
     changeSelectionAfterCommand(
         newSelection,
         FrameSelection::CloseTyping | FrameSelection::ClearTypingStyle);
+  }
 
   m_lastEditCommand = nullptr;
   m_undoStack->registerRedoStep(cmd);