Do not dispatch 'change' event if the INPUT value is not changed.

This CL fixes two problems.

* When the INPUT type is changed, m_textAsOfLastFormControlChangeEvent is not
  initialized correctly.  So, we dispatched unexpected change events.

* When readonly/disabled state is changed, we call
  SpinButtonElement::releaseCapture(), which can dispatch a 'change' event. Its
  callsite should protect |this|.


Review URL:

git-svn-id: svn:// bbb929c8-8fbe-4397-9dbb-9b2b20218538
6 files changed