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}
DEFINE_TRACE(Editor) {
  visitor->trace(m_frame);
  visitor->trace(m_lastEditCommand);
  visitor->trace(m_undoStack);
  visitor->trace(m_mark);
  visitor->trace(m_typingStyle);
}

DEFINE_TRACE(Editor) {
  visitor->trace(m_frame);
  visitor->trace(m_lastEditCommand);
  visitor->trace(m_undoStack);
  visitor->trace(m_mark);
  visitor->trace(m_typingStyle);
}
