Implement RenderThemeChromiumDefault::paintMenuListButton.

We didn't have paintMenuListButton implenetation, and the parent class
RenderThemeChromiumSkia just calls paintMenuList. We should implement
paintMenuListButton correctly.

The new paintMenuListButton implmentation is a copy of paintMenuList,
but it doesn't render anything but the arrow button.

BUG=335472,336364
TEST=automated

Review URL: https://codereview.chromium.org/149243004

git-svn-id: svn://svn.chromium.org/blink/trunk@166061 bbb929c8-8fbe-4397-9dbb-9b2b20218538