MediaVolumeSliderPart is not vertical

This makes some parts of the keymapping a bit more intuitive for the
volume-slider of media controls (-webkit-appearance: media-volume-slider).

BUG=135661

Review URL: https://codereview.chromium.org/294963003

git-svn-id: svn://svn.chromium.org/blink/trunk@174503 bbb929c8-8fbe-4397-9dbb-9b2b20218538