input[type=range] should release mouse event capturing when it is disabled.

We should call SliderThumbElement::stopDragging() when the host input element
is disabled.

SliderThumbElement::defaultEventHandler() calls stopDragging() if the input
element is disabled.  But it makes no sense because a disabled elemnt doesn't
receive any events.

* SliderThumbElement::dragFrom:
  We need to change the order of setPositionFromPoint() and startDragging()
  because input/change events can disable the input and we don't want to call
  startDragging after it.

BUG=326406
TEST=An automated test is added.

Review URL: https://codereview.chromium.org/99393003

git-svn-id: svn://svn.chromium.org/blink/trunk@163325 bbb929c8-8fbe-4397-9dbb-9b2b20218538