SliderThumb is responsible for sending onchange for range input type. It is updated to send change event in its defaultEventHandler only for mouse up event. 
Previously onchange was triggered for any position change

R=tkent             
BUG=155747
TEST=Range toggle test updated to generate onchange only on mouseUp.

Review URL: https://codereview.chromium.org/125973004

git-svn-id: svn://svn.chromium.org/blink/trunk@165061 bbb929c8-8fbe-4397-9dbb-9b2b20218538