Move Editor::Transpose() out of Editor class

This patch moves |Editor::Transpose()| out of |Editor| class as preparation of
expanding it into |ExecutTranspose()| in "EditorCommand.cpp" to make |Editor|
class simpler for improving code health.

Following patch will expand |Transpose()| into |ExecutTranspose()|.

Bug: 672405
Change-Id: Icde253623f31813d2b4517c4da7d4798bd5fadf6
Reviewed-on: https://chromium-review.googlesource.com/583880
Reviewed-by: Xiaocheng Hu <xiaochengh@chromium.org>
Commit-Queue: Yoshifumi Inoue <yosin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#489518}
static bool ExecuteInsertLineBreak(LocalFrame& frame,
                                   Event* event,
                                   EditorCommandSource source,
                                   const String&) {
  switch (source) {
    case kCommandFromMenuOrKeyBinding:
      return TargetFrame(frame, event)
          ->GetEventHandler()
          .HandleTextInputEvent("\n", event, kTextEventInputLineBreak);
    case kCommandFromDOM:
      DCHECK(frame.GetDocument());
      return TypingCommand::InsertLineBreak(*frame.GetDocument());
  }
  NOTREACHED();
  return false;
}

static bool ExecuteInsertLineBreak(LocalFrame& frame,
                                   Event* event,
                                   EditorCommandSource source,
                                   const String&) {
  switch (source) {
    case kCommandFromMenuOrKeyBinding:
      return TargetFrame(frame, event)
          ->GetEventHandler()
          .HandleTextInputEvent("\n", event, kTextEventInputLineBreak);
    case kCommandFromDOM:
      DCHECK(frame.GetDocument());
      return TypingCommand::InsertLineBreak(*frame.GetDocument());
  }
  NOTREACHED();
  return false;
}
