Remove RenderTheme::usesVerticalVolumeSlider().

RenderThemeChromiumSkia and RenderThemeChromiumMac always return false, which means we can eliminate calling code that handles the true case.

BUG=337015

Review URL: https://codereview.chromium.org/145003007

git-svn-id: svn://svn.chromium.org/blink/trunk@165715 bbb929c8-8fbe-4397-9dbb-9b2b20218538