Add 9 Patch Scrollbar into Blink ScrollbarThemeOverlay

Blink used to only paint solid color scrollbars for Overlay scrollbars.
This patch added code path to call native theme aura to paint 9 patch
scrollbar when there is no color set for ScrollbarThemeOverlay.

BUG=307578

Review URL: https://codereview.chromium.org/193433002

git-svn-id: svn://svn.chromium.org/blink/trunk@169190 bbb929c8-8fbe-4397-9dbb-9b2b20218538