Remove lots of useless RenderTheme code

RenderTheme has way too many hooks that do nothing or virtual functions
that return fixed values and are never overriden. This patch deletes all
that. We can always add back the hooks for the various UI parts in the
future when some platform needs that level of customization.

Review URL: https://codereview.chromium.org/48903005

git-svn-id: svn://svn.chromium.org/blink/trunk@160749 bbb929c8-8fbe-4397-9dbb-9b2b20218538