Remove RenderTheme::supportsClosedCaptioning().

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

BUG=337015

Review URL: https://codereview.chromium.org/145553008

git-svn-id: svn://svn.chromium.org/blink/trunk@165731 bbb929c8-8fbe-4397-9dbb-9b2b20218538