Add a runtime feature to enable overlay scrollbars on Win and Linux.

This is very experimental. I renamed the ScrollbarThemeChromiumAndroid to
ScrollbarThemeChromiumOverlay and let the runtime switch allow usage of
the overlay scrollbars on Aura and Linux. This has no impact on OSX and
does not change the behavior on Android.

I wanted to use a Setting (to allow runtime switching, e.g., when plugging
in a mouse), but that's tricky because ScrollbarTheme::nativeTheme is a
global.

BUG=None

Review URL: https://chromiumcodereview.appspot.com/22636002

git-svn-id: svn://svn.chromium.org/blink/trunk@155814 bbb929c8-8fbe-4397-9dbb-9b2b20218538