Make TypingCommand::insertText() to take SelectionInDOMTree instead of VisibleSelection

This patch makes |TypingCommand::insertText()| to take |SelectionInDOMTree|
instead of |VisibleSelection| to reduce usage of |VisibleSelection| for
improving code health.

BUG=657237
TEST=n/a

Review-Url: https://codereview.chromium.org/2733183002
Cr-Commit-Position: refs/heads/master@{#455368}
void TypingCommand::insertParagraphSeparator(EditingState* editingState) {
  if (!canAppendNewLineFeedToSelection(endingSelection()))
    return;

  applyCommandToComposite(InsertParagraphSeparatorCommand::create(document()),
                          editingState);
  if (editingState->isAborted())
    return;
  typingAddedToOpenCommand(InsertParagraphSeparator);
}

void TypingCommand::insertParagraphSeparator(EditingState* editingState) {
  if (!canAppendNewLineFeedToSelection(endingSelection()))
    return;

  applyCommandToComposite(InsertParagraphSeparatorCommand::create(document()),
                          editingState);
  if (editingState->isAborted())
    return;
  typingAddedToOpenCommand(InsertParagraphSeparator);
}
