Make scrollbar thumb thickness and margin configurable.

When using overlay scrollbars on desktop, remove the margin and
increase the width.  This makes them easier to see on desktop.
No visible changes to android.

BUG=274010
TEST=None

Review URL: https://chromiumcodereview.appspot.com/22815011

git-svn-id: svn://svn.chromium.org/blink/trunk@156482 bbb929c8-8fbe-4397-9dbb-9b2b20218538