Remove WebThemeEngine::paintScrollbarThumb.
Remove WebThemeEngine::paintScrollbarThumb.

BUG=327471

Review URL: https://codereview.chromium.org/644223002

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@183581 bbb929c8-8fbe-4397-9dbb-9b2b20218538

primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs