Remove sliderThumbElement() and sliderTrackElement of HTMLInputElement and InputType.

Also remove sliderThumbElementOf() and sliderTrackElementOf().
We use userAgentShadowRoot()->getElementById() instead.

BUG=

Review URL: https://chromiumcodereview.appspot.com/23011014

git-svn-id: svn://svn.chromium.org/blink/trunk@156145 bbb929c8-8fbe-4397-9dbb-9b2b20218538