Don't let RenderScrollbarPart have padding.

Return 0 padding for all sides. While RenderScrollbarPart isn't laid
out normally and padding is ignored anyway, the padding getters may
still be used from outside layout, and if padding is percent-based, a
containing block is required, which not all RenderScrollbarPart
objects have, so it would crash.

BUG=341004

Review URL: https://codereview.chromium.org/138903031

git-svn-id: svn://svn.chromium.org/blink/trunk@167640 bbb929c8-8fbe-4397-9dbb-9b2b20218538